Unit 5: The Lymphatic System, Resistance & Immunity

Part 1: The Lymphatic System

Anatomy of the Lymphatic System

The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and that filters pathogens from the blood and tissues. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between the lymphatic and immune system (discussed later in part 2).

Functions of the Lymphatic System: A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between individual cells in the tissues. In humans, 20 litres of plasma are released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 litres are reabsorbed directly by the blood vessels. But what happens to the remaining three litres? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences, such as increased risk of infection, pain and disability.

As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, dietary lipids and fat-soluble vitamins absorbed in the gut use this system of transport.

Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses to fight off infection from the body’s tissues. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.

Structure of the Lymphatic System: The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body (Figure 1).

Lymphatic Capillaries: Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Due to the permeability characteristics of these vessels compared to blood capillaries, they preferentially take up the fluid, proteins and any potential pathogens and cell debris that may be present in the tissue. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body (Figure 2). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymphatic vessels.

 

Larger Lymphatic Vessels, Trunks, and Ducts: The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three—layered walls and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 2). The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks. On the right side of the body, the right sides of the head, thorax, and the right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct (Figure 3). On the left side of the body, the remaining portions of the body drain lymph into the larger thoracic duct, which drains into the left subclavian vein.

 

Primary Lymphoid Organs and Lymphocyte Development

Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response (described later). The primary lymphoid organs are the bone marrow and thymus gland. It is in these organs where lymphocytes originate, proliferate and mature. It is through this maturation process that the lymphocytes learn to destroy pathogens and leave the body’s own healthy cells unharmed, a phenomenon referred to as immunological or self tolerance. Failure of the body’s lymphocytes to develop self-tolerance leads to autoimmune diseases, which will be discussed later.

Bone Marrow: The red bone marrow is where blood cell development or hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, consisting largely of fat cells (Figure 4). The B lymphocyte (B cell) undergoes nearly all of its development in the red bone marrow, whereas the immature T lymphocyte (T cell), called a thymocyte, leaves the bone marrow and matures largely in the thymus gland.

 

Thymus: The thymus gland is a bi-lobed organ found in the space posterior to the sternum and anterior to the heart, it overlies the aortic arch, superior vena cava and trachea. The organ contains large numbers of thymocytes along with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). As mentioned, thymocytes mature into T cells in the thymus.

Secondary Lymphoid Organs and their Roles in Active Immune Responses: Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs. A naïve lymphocyte is one that has left the primary lymphoid organ and entered a secondary lymphoid organ. Naïve lymphocytes, though mature have yet to encounter their matching antigen, when stimulated by this antigen they become fully functional immunologically. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules, such as the tonsils.

Lymph Nodes: Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph”. Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells as described later in the chapter.

Tonsils: These lymphoid nodules located along the inner surface of the pharynx are important in developing immunity to oral pathogens (Figure 5). The tonsil located at the back of the throat, called the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/ or swallowing.

 

Watch this CrashCourse video to learn more about the lymphatic system! Direct link: https://youtu.be/I7orwMgTQ5I

Part 2: The Immune System

In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii).

Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen. This is a pathogen which causes disease in individuals with suppressed or underdeveloped immune systems; the very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible. The five patients from LA, though, were between 29 and 36 years of age and should have been in the prime of their lives, immunologically speaking. What could be going on?

A few days later, a cluster of eight cases was reported in New York City, also involving young adult patients, this time exhibiting a rare form of skin cancer known as Kaposi’s sarcoma. This cancer of the cells that line the blood and lymphatic vessels was previously observed as a relatively innocuous disease of the elderly. The disease that doctors saw in 1981 was frighteningly more severe, with multiple, fast-growing lesions that spread to all parts of the body, including the trunk and face. Could the immune systems of these young patients have been compromised in some way? Indeed, when they were tested, they exhibited extremely low numbers of a specific type of white blood cell in their bloodstreams, indicating that they had somehow lost a major part of the immune system.

Acquired immune deficiency syndrome, or AIDS, turned out to be a new disease caused by the previously unknown human immunodeficiency virus (HIV) which infects and slowly destroys Helper T cells. Although nearly 100 percent fatal in those with active HIV infections in the early years, the development of anti-HIV drugs has transformed HIV infection into a chronic, manageable disease and not the certain death sentence it once was. One positive outcome resulting from the emergence of HIV disease was that the public’s attention became focused as never before on the importance of having a functional and healthy immune system.

The Organization of Immune Function

The immune system is a functional system rather than an anatomical system; it contains aspects of the lymphatic, cardiovascular, integumentary and skeletal systems.   It is the complex collection of barriers, cells, soluble proteins and organs that interact with each other in extraordinary ways to destroy or neutralize pathogens that would otherwise cause disease or death.  The immune system can be divided into two overlapping mechanisms to destroy pathogens: the innate immune response, which is relatively rapid but nonspecific and unable to change its response to particular pathogens and the adaptive immune response, which is slower in its development following infection, but is highly specific and effective at attacking a wide variety of pathogens (Figure 6). The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following:

• Barrier defenses of the innate immune system, such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues.
• The rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors.
• The slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses.

Barrier Defenses and the Innate Immune Response

Any discussion of the innate immune response usually begins with the physical barriers that prevent pathogens from entering the body, destroy them after they enter, or flush them out before they can establish themselves in the hospitable environment of the body’s soft tissues. Barrier defenses are part of the body’s most basic defense mechanisms. The barrier defenses are not a response to infections, but they are continuously working to protect against a broad range of pathogens.

The different modes of barrier defenses are associated with the external surfaces of the body, where pathogens may try to enter (Table 1). The primary barrier to the entrance of microorganisms into the body is the skin. Not only is the skin covered with a layer of dead, keratinized epithelium that is too dry for bacteria in which to grow, but as these cells are continuously sloughed off from the skin, they carry bacteria and other pathogens with them. Additionally, sweat can physically wash microbes away and it, along with other skin secretions contain chemical factors to inhibit growth or destroy microbes, for example toxic lipids and acidic molecules from sebum and antimicrobial peptides, such as dermcidin.

Another barrier is the saliva which washes the mouth and teeth and is rich in lysozyme—an enzyme that destroys bacteria by digesting their cell walls. The acidic environment of the stomach, which is fatal to many pathogens, is also a barrier. Additionally, the mucus layer of the gastrointestinal tract, respiratory tract, reproductive tract, eyes, ears, and nose traps both microbes and debris, and facilitates their removal. In the case of the upper respiratory tract, ciliated epithelial cells move potentially contaminated mucus upwards to the mouth, where it is then swallowed into the digestive tract, ending up in the harsh acidic environment of the stomach. Considering how often you breathe compared to how often you eat or perform other activities that expose you to pathogens, it is not surprising that multiple barrier mechanisms have evolved to work in concert to protect this vital area.

Table 1: Barrier Defenses

Site	Defensive structure	Protective aspect
Skin (physical structure)	Epidermal surface	Keratinized cells of surface, Langerhans cells
Skin (secretions)	Eccrine glands	Low pH, dermcidin, washing action
Oral cavity	Salivary glands	Lysozyme
Stomach	Gastric juice	Low pH
Mucous membranes	Mucosal epithelium	Layered cells
Mucous membranes (secretions)	Cells producing mucus	Traps pathogens, dust, debris, etc.; washing action; defensins and lysozyme
Skin and mucosal surfaces	Normal flora (nonpathogenic bacteria)	Compete with pathogenic microbes

Cells of the Innate Immune Response

Phagocytes: A phagocyte is a cell that is able to surround and engulf a particle or cell and digest it internally in a process called phagocytosis. Phagocytosis is an important and effective mechanism of destroying pathogens during innate immune responses. The major phagocytes of the immune system are the leukocytes, macrophages, neutrophils and dendritic cells (Table 2). These engulf other particles or cells into a vesicle and destroy them using lysosomal enzymes, either to clean an area of debris, remove old cells, or to kill pathogenic organisms such as bacteria. The phagocytes are the body’s fast acting, first line of immunological defense against organisms that have breached barrier defenses and have entered the vulnerable tissues of the body.

A monocyte is a circulating precursor cell that differentiates into either a macrophage or dendritic cell in tissues. Monocytes can be rapidly attracted to areas of infection by signal molecules of inflammation.

A macrophage is an irregularly shaped phagocyte that is amoeboid in nature and is the most versatile of the phagocytes in the body. Macrophages move through tissues and squeeze through capillary walls using pseudopodia. They not only participate in innate immune responses to destroy pathogens but have also evolved to cooperate with lymphocytes, as antigen-presenting cells (APCs) as part of the adaptive immune response (discussed later in the chapter). Macrophages exist in many tissues of the body, either freely roaming through connective tissues or fixed to reticular fibres within specific tissues such as lymph nodes. They are called different names, depending on the tissue: Kupffer cells in the liver, histiocytes in connective tissue, microglia in the brain, and alveolar macrophages in the lungs.

A dendritic cell is a phagocytic cell found in particular tissues and organs, including secondary lymphoid organs. In tissues they have contact with the outside environment and for part of the skin and mucous membranes. Their primary role is to function as antigen-presenting cells (APCs) which interact with lymphocytes to stimulate the adaptive immune response.

A neutrophil is a phagocytic cell that is attracted via chemotaxis from the bloodstream to infected tissues. Whereas macrophages act like sentries, always on guard against infection, neutrophils can be thought of as military reinforcements that are called into a battle to hasten the destruction of the enemy. Neutrophils are usually thought of as the primary pathogen-killing cell of the inflammatory process of the innate immune response.

Table 2: Phagocytic Cells of the Innate Immune System

Cell	Cell type	Primary location	Function in the innate immune response
Macrophage	Agranulocyte	Body cavities/organs	Phagocytosis
Dendritic cell	Agranulocyte	Skin and mucous membranes	Phagocytosis
Neutrophil	Granulocyte	Blood	Phagocytosis
Monocyte	Agranulocyte	Blood	Precursor of macrophages and dendritic cells

Soluble Mediators of the Innate Immune Response: During innate responses and later during adaptive immune responses immune cells and damaged cells secrete soluble signaling proteins such as cytokines or chemokines to recruit and activate immune cells. For example, tumor necrosis factor (TNF) from macrophages increases inflammation and fever. Proteins involved in the complement system are also important mediators of the innate immune response. The complement system is a series of plasma proteins which are recruited to sites of infection and become activated in signaling cascades with various outcomes, such as, labeling pathogens for phagocytosis (opsonization), killing pathogens by directly damaging their plasma membrane, and intensifying inflammation

Inflammatory Response: Everyone has experienced inflammation at some point in their lives. Stub a toe, cut a finger, or perform any activity that causes tissue damage and inflammation will result, with its four characteristics: heat, redness, pain, and swelling (“loss of function” is sometimes mentioned as a fifth characteristic). It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries; the inflammatory response is the same irrespective of the cause of tissue damage. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter (by hitting your thumb with a hammer, for example). The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and to set the stage for wound repair (Figure 7).

 

This reaction also brings in the cells of the innate immune system, allowing them to get rid of the sources of a possible infection or injury. Inflammation is part of the innate immune response. The process not only brings fluid and cells into the site of damage to destroy the pathogen and remove it and any debris from the site, but it also helps to isolate the site, limiting the spread of the pathogen. Acute inflammation is a short-term inflammatory response to an insult to the body. If the cause of the inflammation is not resolved, however, it can lead to chronic inflammation, which is associated with major tissue destruction and fibrosis. Chronic inflammation is ongoing inflammation. It can be caused by foreign bodies, persistent pathogens, and autoimmune diseases such as rheumatoid arthritis.

There are four important parts to the inflammatory response:

• Tissue Injury. The released contents of injured cells stimulate the release of mast cell granules and their potent inflammatory mediators, which have the following downstream effects.
• Vasodilation. Many inflammatory mediators, such as histamine, are vasodilators that increase the diameter of local capillaries. This causes increased blood flow and is responsible for the heat and redness of inflamed tissue. It allows greater access of the immune components of blood to the site of inflammation.
• Increased Vascular Permeability. At the same time, inflammatory mediators, like histamine and some complement proteins, increase the permeability of the local vasculature, causing leakage of fluid into the interstitial space, resulting in the swelling, or edema, associated with inflammation. This allows immune cells and mediators to exit the blood stream and enter the site of infection or injury.
• Recruitment of Phagocytes. Inflammatory mediators also attract neutrophils from the blood to the site of infection by chemotaxis. Following an early neutrophil infiltration stimulated by macrophage signals, more macrophages are recruited to clean up the debris remaining at the site. When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site.

Overall, inflammation is valuable for many reasons. Not only are the pathogens killed and debris removed, but the increase in vascular permeability encourages the entry of clotting factors, the first step towards wound repair. Inflammation also facilitates the transport of antigen to lymph nodes by macrophages or dendritic cells for the development of the adaptive immune response.

Fever: The mechanisms of inflammation described so far are primarily local. Another inflammatory response that is systemic in nature is that of fever. Fever is defined as an increase in the set-point of the body’s thermostat, with the result that homeostatic mechanisms raise the temperature of the body above the normal of about 37ºC.

The increase in temperature has several effects that are beneficial to the body’s defense. These include increasing the activity of the immune system (e.g., enhancing the efficiency of white blood cells). Fever also reduces the rate of growth of microbes. The beneficial effects of such an increase in body temperature disappear, however, should the value go over 41ºC, as human proteins begin denaturing.

The Adaptive Immune Response

Primary Disease and Immunological Memory: Immunological memory is another benefit of adaptive immunity. The immune system’s first exposure to a pathogen is called a primary adaptive response or primary immune response. Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.

Upon re-exposure to the same pathogen, a secondary adaptive response or secondary immune response is generated, which is stronger and faster than the primary response. The secondary immune response often eliminates a pathogen before it can cause significant tissue damage or any symptoms. Without symptoms, there is no disease, and the individual is not even aware of the infection. This secondary response is due to the production of a type of B and T cells, known as memory cells (discussed below). These memory cells form the basis of immunological memory, which protects us from getting diseases repeatedly from the same pathogen. By this mechanism, an individual’s exposure to pathogens early in life spares the person from these diseases later in life

B and T lymphocytes – Antigen binding and Activation: Despite their differences in function, both B and T cells are responsive to antigen and develop in similar ways. Naïve B and T cells (those that have matured in primary lymphoid tissue but have not yet encountered their matching antigen) become activated when they first recognize a specific foreign antigen in lymph nodes or other secondary lymphoid tissue. The antigen binds to the specific surface receptors on the B or T cells and selects that cell for further development (clonal selection). Only those clones of lymphocytes whose receptors are activated by the antigen are stimulated to proliferate rapidly and complete their differentiation. This proliferation of lymphocytes is called clonal expansion and is necessary to generate large numbers of specific lymphocytes to make the immune response strong enough to effectively control a pathogen. The clones, each with their unique receptor, differentiate into both effector and memory cells. The effector cells (also known as “activated” lymphocytes) will fight the current infection and the memory cells will be recruited in subsequent infections to generate the secondary immune response (see Figure 8). Memory cells are long-lived and can even persist for a lifetime. Memory cells are primed to act rapidly. Thus, any subsequent exposure to the pathogen will elicit a very rapid B or T cell response. This rapid, secondary immune response generates large numbers of effector B or T cells from memory cells so fast that the pathogen is often overwhelmed before it can cause any symptoms of disease. This is what is meant by immunity to a disease.

T Cell-Mediated Immune Responses: T cells are particularly important, as they not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well. Thus, many of the decisions about how to attack a pathogen are made at the T cell level, and knowledge of their functional types is crucial to understanding the functioning and regulation of adaptive immune responses as a whole.

Antigen-presenting Cells: T cells can only respond to antigens that are presented to them on the surface of body cells; for helper T-cells, this is the job of so-called antigen-presenting cells (APCs). APCs represent an important link between the innate and adaptive immune response. These stimulators of the cell-mediated response include macrophages, dendritic cells, and B cells. These APCs engulf the pathogen, destroy it and display a small component of the processed pathogen (an antigen) on their surface. Dendritic cells and macrophages engulf pathogens in interstitial tissues and bring antigens to regional draining lymph nodes to “present” them to the T cells to mount an immune response. B cells also present antigens to T cells, to allow full activation of B cells as described later. For cytotoxic T cells, antigens are presented to them on the surface of infected cells or abnormal host cells, such as tumour cells.

T Cell Types and their Functions: T cells can contain cell adhesion molecules that keep the T cell in close contact with the antigen-presenting cell by directly binding to the antigen-presenting receptor (MHC class I or MHC class II) on its membrane. There are two main types of T cell based on these cell adhesion molecules: either CD4 or CD8 molecules (CD refers to Cluster of Differentiation).) Although the correlation is not absolute, CD4-bearing T cells are associated with helper functions and CD8-bearing T cells are associated with cytotoxicity.

Helper T Cells and their Cytokines: Following their interaction with antigen on the surface of APCs, activated helper T cells (Th), bearing the CD4 molecule, will fight infection by secreting cytokines that act to enhance other immune responses (Figure 9). There are two classes of Th cells, and they act on different components of the immune response. These cells are not distinguished by their surface molecules but the characteristic set of cytokines they secrete.

Th1 cells are a type of helper T cell that secretes cytokines that regulate the activity and development of a variety of cells, including macrophages and other types of T cells.

Th2 cells, on the other hand, are cytokine-secreting cells that act on B cells to drive their differentiation into plasma cells that make antibody. The helper T cells therefore play an integral role in the immune response by stimulating cells involved in both the innate and adaptive immunity.

Cytotoxic T cells: Activated cytotoxic T cells (Tc) bearing the CD8 molecules are T cells that migrate to the site of infection to target abnormal cells in the body, such as virus-infected cells, cancer cells and transplanted cells. Abnormal body cells are recognized by cytotoxic T cells because they carry foreign antigens on their surface. Cytotoxic T cells kill these abnormal cells by inducing their apoptosis, that is, programmed cell death using cytotoxic (cell-killing) granules). In addition, as long as the antigen is recognized by the cell, each Tc cell can kill more than one target cell, making them especially effective. While they are active in all pathogenic infections, Tc cells are so important in the antiviral immune response that some speculate that this was the main reason the adaptive immune response evolved in the first place.

B-cell Mediated Humoral Immune Response and Antibodies: As previously described activation of naïve B cells occurs when the cells with appropriate antigen specificity bind to their matching antigen. This leads to selection and expansion of that clone and simultaneous differentiation into plasma cells (effector B cells) and memory B cells (Figure 10). Unlike T cells, B cells respond to antigens present in the extracellular fluids, such as intact bacteria or viruses or soluble, foreign material, rather than antigens presented to them on the surface of body cells. The plasma cells secrete antibodies with antigenic specificity identical to those that were on the surfaces of the selected B cells. Full activation of B cells requires interaction with helper T cells and stimulation by certain cytokines (Figure 14). After secreting antibodies for a specific period, plasma cells die, as most of their energy is devoted to making antibodies and not to maintaining themselves.

 

 

Memory B cells function in a way similar to memory T cells as described earlier: they lead to a stronger and faster secondary immune response when compared to the primary immune response. These immune responses can be followed in B cells because antibodies are easily measured in blood samples, (Figure 11). The primary response to an antigen is delayed by several days. This is the time it takes for the B cell clones to expand and differentiate into plasma cells. The level of antibody produced is low, but it is sufficient for immune protection. The second time a person encounters the same antigen, there is no time delay, the memory cells, on binding the antigen, quickly differentiate into plasma cells which are more efficient at producing antibody. The amount of antibody is higher compared to the primary response. Thus, the secondary antibody response overwhelms the pathogens quickly and, in most situations, no symptoms are felt. When a different antigen is used, another primary response is made with its low antibody levels and time delay.

 

Antibody Structure and Function: Antibodies were the first component of the adaptive immune response to be characterized by scientists working on the immune system. It was already known that individuals who survived a bacterial infection were immune to re-infection with the same pathogen. Early microbiologists took serum from an immune patient and mixed it with a fresh culture of the same type of bacteria, then observed the bacteria under a microscope. The bacteria became clumped in a process called agglutination. When a different bacterial species was used, the agglutination did not happen. Thus, there was something in the serum of immune individuals that could specifically bind to and agglutinate bacteria. Scientists now know the cause of the agglutination is an antibody molecule.

What is an antibody? An antibody, also known as an immunoglobulin (Ig), is essentially a secreted form of a B cell surface receptor and is released by activated B cells called plasma cells. The antibodies of the plasma cell have the exact same antigen-binding site and specificity as their B cell precursors.

Antibody Structure: Antibodies are glycoproteins consisting of two types of polypeptide chains, the heavy chain and the light chain; two of each are required to form a generic antibody structure (Figure 12).  The light chains have an important role, forming part of the antigen-binding site on the antibody molecules. There are five different classes of antibody found in humans: IgM, IgD, IgG, IgA, and IgE. Each class differs in their heavy chain structure, and has their own location and specific functions in the immune response.

Functions of Antibodies: In general, antibodies have two basic functions. They can act as the B cell antigen receptor (on the B cell surface) or they can be secreted, circulate, and bind to a pathogen, often labeling it for identification by other components of the immune response.

 

An antibody bound to an antigen molecule on the surface of a pathogen can enhance the phagocytosis of the pathogen, a phenomenon known as opsonization. It can also fix and activate the complement system, a series of signaling cascades which lead to an enhancement of phagocytosis, a local inflammatory response, and lysis of the pathogen. Note that antibodies do not destroy pathogens themselves but are involved in tagging pathogens for removal by other components of the immune system, particularly the innate immune response.

 

 

 

The adaptive immune response is summarized in Figure 14. Note the different ways B and T cells are activated and how the cytokines from the Helper T cells activate cells of both the humoral and cell-mediated responses, and the innate immune system.

Active versus Passive Immunity

Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by (1) the active development of an immune response in the infected individual or (2) the passive transfer of antibodies from an immune individual to a nonimmune one. Both active and passive immunity have examples in the natural world and as part of medicine.

Active immunity is the resistance to pathogens acquired during an adaptive immune response within an individual (Figure 15). Naturally acquired active immunity, the response to a pathogen, is the focus of this section on adaptive immunity. Artificially acquired active immunity involves the use of vaccines. A vaccine is a killed or weakened (attenuated) pathogen or its components that, when administered to a healthy individual, leads to the development of memory cells and immunological memory (via a primary immune response) without causing much in the way of symptoms. Killed vaccines or inactivated vaccines consist of pathogens that have been killed and so are no longer viable, but still retain antigens that can be recognized and used to mount an immune response. Live attenuated vaccines, like the one used against the measles virus, involve the use of pathogens (mainly viruses) which have been rendered harmless or less virulent so they do not reproduce and spread inside an individual. Toxoid vaccines include a toxin molecule (a poison secreted by bacteria) that has been modified to be harmless to human cells but still elicits an immune response against the toxin. The tetanus vaccine for example contains a modified version of the toxin tetanospasmin that is normally released by the bacterium Clostridium tetani and causes muscle spasms. This vaccine triggers the production of anti-tetanospasmin antibodies that confer immunity to the live bacterium’s harmful effects. Subunit vaccines are more modern types of vaccine which expose the patient to small components of a pathogen—not whole cells or viruses. Subunit vaccines can be prepared either by chemically degrading a pathogen and isolating its key antigens or by producing the antigens through genetic engineering. This type of vaccine is commonplace nowadays (for example the HPV, hepatitis B and influenza vaccines). They do not contain the whole pathogen, just the essential antigens, therefore the risk of side effects is relatively low.

The recent COVID-19 pandemic has seen the development and widespread use of nucleic acid vaccines in which genetic instructions to build a small component of the SARS-Cov-2 virus (spike protein) are delivered into an individual; either in the form of a gene (double stranded DNA) or messenger RNA (mRNA). The genetic material is packaged either into a harmless virus (adenovirus, for DNA) or into lipid nanoparticles (for mRNA) which aids uptake into the individual’s cells where the instructions are read and the foreign viral spike protein is produced. These vaccines can be prepared relatively cheaply and quickly and effectively stimulate both humoral and cell mediated immune responses. As a result, this method of vaccine preparation is in development for many diseases like, malaria and HIV.

A person can also acquire protection from specific pathogens through the administration of pre-formed antibodies, known as passive immunity. Naturally acquired passive immunity is represented by the transfer of antibodies in breast milk or through the placenta that give newborn babies protection against some pathogens as they are developing their own immune response. There is a particular type of breast milk produced by the mother 48- 72 hours after delivery, called colostrum, which is highly enriched in antibodies to help the newborn rapidly establish immunity after the exposure to the non-sterile environment outside the womb. The breast milk matures to contain more lipids and glucose for growth, but still retains antibody content from the mother. In medicine, artificially acquired passive immunity usually involves injections of immunoglobulins taken from animals previously exposed to a specific pathogen. This treatment is a fast-acting method of temporarily protecting an individual who was possibly exposed to a pathogen. The downside to this treatment is the lack of the development of immunological memory. Once the antibodies are transferred, they are effective for only a limited time before they degrade, so multiple injections may be necessary. An example is the treatment of suspected rabies with postexposure prophylaxis that includes immunoglobulin injections.

From the above, it is readily apparent that with the use of vaccines, one can avoid the damage from disease that results from the first exposure to the pathogen, yet reap the benefits of protection from immunological memory. The advent of vaccines was one of the major medical advances of the twentieth century and led to the eradication of smallpox and the control of many infectious diseases, including polio, measles, and whooping cough.

 

Watch this Crash Course video for an overview of the adaptive immune response! Direct link: https://youtu.be/2DFN4IBZ3rI

Diseases Associated with Depressed or Overactive Immune Responses

This section is about how the immune system goes wrong. When it goes haywire, and becomes too weak or too strong, it leads to a state of disease. The factors that maintain immunological homeostasis are complex and incompletely understood.

Immunodeficiencies: As you have seen, the immune system is quite complex. It has many pathways using many cell types and signals. Because it is so complex, there are many ways for it to go wrong, and in the case of immunodeficiencies, become weakened. Inherited immunodeficiencies arise from gene mutations that affect specific components of the immune response. There are also acquired immunodeficiencies that result from causes other than inheritance with potentially devastating effects on the immune system, such as infection with HIV.

Inherited Immunodeficiencies: A list of all inherited immunodeficiencies is well beyond the scope of this book. The list is almost as long as the list of cells, proteins, and signaling molecules of the immune system itself. Some deficiencies, such as those for complement, cause only a higher susceptibility to some Gram- negative bacteria. Others are more severe in their consequences. Certainly, the most serious of the inherited immunodeficiencies is severe combined immunodeficiency disease (SCID). This disease is complex because it is caused by many different genetic defects. What groups them together is the fact that both the B cell and T cell arms of the adaptive immune response are affected. Children with this disease usually die of opportunistic infections within their first year of life unless they receive a bone marrow transplant.

Human Immunodeficiency Virus/AIDS: Although many viruses cause suppression of the immune system, only one wipes it out completely, and that is the previously mentioned HIV. The virus is transmitted through semen, vaginal fluids, and blood. There are sometimes, but not always, flu-like symptoms in the first 1 to 2 weeks after infection. Following this time (with no medical intervention), the levels of CD4+ cells, especially helper T cells, decline steadily, eventually producing an acquired immunodeficiency syndrome (AIDS), until at some point, the immune response is so weak that opportunistic disease (a condition that does not affect those with a healthy immune system) and eventually death result. CD4 is the receptor that HIV uses to get inside T cells and reproduce. Given that CD4+ helper T cells play an important role in T cell immune responses and antibody responses, it should be no surprise that both types of cellular and humoral immune responses are eventually seriously compromised.

Treatment for the disease consists of drugs that target virally encoded proteins that are necessary for viral replication but are absent from normal human cells. By targeting the virus itself and sparing the cells, this approach has been successful in significantly prolonging the lives of HIV-positive individuals. On the other hand, an HIV vaccine has been 30 years in development and is still years away. Because the virus mutates rapidly to evade the immune system, scientists have been looking for parts of the virus that do not change and thus would be good targets for a vaccine candidate.

Autoimmune Responses: The worst cases of the immune system over-reacting are autoimmune diseases. Somehow, immunological tolerance breaks down and the immune systems in individuals with these diseases begin to attack their own bodies, causing significant damage. The trigger for these diseases is, more often than not, unknown, and the treatments are usually based on resolving the symptoms using immunosuppressive and anti- inflammatory drugs such as steroids. These diseases can be localized and crippling, as in rheumatoid arthritis, or diffuse in the body with multiple symptoms that differ in different individuals, as is the case with systemic lupus erythematosus (Figure 16).

Part 3: Blood Typing

Blood transfusions in humans were risky procedures until the discovery of the major human blood groups by Karl Landsteiner, an Austrian biologist and physician, in 1900. Until that point, physicians did not understand that death sometimes followed blood transfusions, when the type of donor blood infused into the patient was incompatible with the patient’s own blood. Blood groups are determined by the presence or absence of specific marker molecules on the plasma membranes of erythrocytes. With their discovery, it became possible for the first time to match patient-donor blood types and prevent transfusion reactions and deaths.

Antigens, Antibodies, and Transfusion Reactions

Antigens are substances that the body does not recognize as belonging to the “self” and therefore trigger a defensive response from the leukocytes of the immune system. Here, we will focus on the role of immunity in blood transfusion reactions. Following an infusion of incompatible blood, erythrocytes with foreign antigens appear in the bloodstream and trigger an immune response. Antibodies produced by the plasma cells, attach to the antigens on the plasma membranes of the infused erythrocytes and cause them to adhere to one another.

• As explained before, because the arms of the Y-shaped antibodies attach randomly to more than one non-self erythrocyte surface, they form clumps of erythrocytes (agglutination).
• The clumps of erythrocytes block small blood vessels throughout the body, depriving tissues of oxygen and nutrients.
• As the erythrocyte clumps are degraded, in a process called hemolysis, their hemoglobin is released into the bloodstream. This hemoglobin travels to the kidneys, which are responsible for filtration of the blood. However, the load of hemoglobin released can easily overwhelm the kidney’s capacity to clear it, and the patient can quickly develop kidney failure.

More than 50 antigens have been identified on erythrocyte membranes, but the most significant in terms of their potential harm to patients are classified into two groups: the ABO blood group and the Rh blood group.

The ABO and Rh Blood Groups

The ABO Blood Group: Although the ABO blood group name consists of three letters, ABO blood typing designates the presence or absence of just two antigens, A and B. Both are glycoproteins. People who have A antigens on their erythrocyte membrane surfaces are designated blood type A, and those whose erythrocytes have B antigens are blood type B. People can also have both A and B antigens on their erythrocytes, in which case they are blood type AB. People with neither A nor B antigens are designated blood type O. ABO blood types are genetically determined.

Normally the body must be exposed to a foreign antigen before an antibody can be produced. This is not the case for the ABO blood group. Individuals with type A blood—without any prior exposure to incompatible blood—have pre-formed antibodies to the B antigen circulating in their blood plasma. These antibodies, referred to as anti-B antibodies, will cause agglutination and hemolysis if they ever encounter erythrocytes with B antigens. Similarly, an individual with type B blood has pre-formed anti-A antibodies. Individuals with type AB blood, which has both antigens, do not have pre-formed antibodies to either of these. People with type O blood lack antigens A and B on their erythrocytes, but both pre-formed anti-A and anti-B antibodies circulate in their blood plasma.

Rh Blood Groups: The Rh blood group is classified according to the presence or absence of a second erythrocyte antigen identified as Rh. (It was first discovered in a type of primate known as a rhesus macaque, which is often used in research, because its blood is similar to that of humans.) Although dozens of Rh antigens have been identified, only one, designated D, is clinically important. Those who have the Rh D antigen present on their erythrocytes—about 85 percent of Americans—are described as Rh positive (Rh+) and those who lack it are Rh negative (Rh−). Note that the Rh group is distinct from the ABO group, so any individual, no matter their ABO blood type, may have or lack this Rh antigen. When identifying a patient’s blood type, the Rh group is designated by adding the word positive or negative to the ABO type. For example, A positive (A+) means ABO group A blood with the Rh antigen present, and AB negative (AB−) means ABO group AB blood without the Rh antigen.

In contrast to the ABO group antibodies, which are preformed, antibodies to the Rh antigen are produced only in Rh− individuals after exposure to the antigen. This process, called sensitization, occurs following a transfusion with Rh-incompatible blood or, more commonly, with the birth of an Rh+ baby to an Rh− mother. Problems are rare in a first pregnancy, since the baby’s Rh+ cells rarely cross the placenta (the organ of gas and nutrient exchange between the baby and the mother). However, during or immediately after birth, the Rh− mother can be exposed to the baby’s Rh+ cells (Figure 17). Research has shown that this occurs in about 13−14 percent of such pregnancies. After exposure, the mother’s immune system begins to generate anti-Rh antibodies. If the mother should then conceive another Rh+ baby, the Rh antibodies she has produced can cross the placenta into the fetal bloodstream and destroy the fetal RBCs. This condition, known as hemolytic disease of the newborn (HDN) or erythroblastosis fetalis, may cause anemia in mild cases, but the agglutination and hemolysis can be so severe that without treatment the fetus may die in the womb or shortly after birth.

A drug known as RhoGAM, short for Rh immune globulin, can temporarily prevent the development of Rh antibodies in the Rh− mother, thereby averting this potentially serious disease for the fetus. RhoGAM antibodies destroy any fetal Rh+ erythrocytes that may cross the placental barrier. RhoGAM is normally administered to Rh− mothers during weeks 26−28 of pregnancy and within 72 hours following birth. It has proven remarkably effective in decreasing the incidence of HDN. Earlier we noted that the incidence of HDN in an Rh+ subsequent pregnancy to an Rh− mother is about 13–14 percent without preventive treatment. Since the introduction of RhoGAM in 1968, the incidence has dropped to about 0.1 percent in the United States.

Determining ABO Blood Types

Clinicians are able to determine a patient’s blood type quickly and easily using commercially prepared antibodies. An unknown blood sample is allocated into separate wells. Into one well a small amount of anti-A antibody is added, and to another a small amount of anti-B antibody. If the antigen is present, the antibodies will cause visible agglutination of the cells (Figure 18). The blood should also be tested with Rh antibodies.

ABO Transfusion Protocols: To avoid transfusion reactions, it is best to transfuse only matching blood types; that is, a type B+ recipient should ideally receive blood only from a type B+ donor and so on. That said, in emergency situations, when acute hemorrhage threatens the patient’s life, there may not be time for cross matching to identify blood type. In these cases, blood from a universal donor—an individual with type O− blood—may be transfused. Recall that type O erythrocytes do not display A or B antigens. Thus, anti-A or anti-B antibodies that might be circulating in the patient’s blood plasma will not encounter any erythrocyte surface antigens on the donated blood and therefore will not be provoked into a response.

A patient with blood type AB+ is known as the universal recipient. This patient can theoretically receive any type of blood, because the patient’s own blood—having both A and B antigens on the erythrocyte surface—does not produce anti-A or anti-B antibodies. In addition, an Rh+ patient can receive both Rh+ and Rh− blood. Figure 19 summarizes the blood types and compatibilities.