The immune system is vitally important in maintaining the health of an individual.  The major tissues and organs of the immune system are involved in producing cells and chemicals that are capable of destroying pathogens and neutralizing toxins that would otherwise contribute to tissue damage or destruction.  This section will focus on the biological components of the immune system, which is closely associated with the lymphatic system.  The lymphatic system provides key structural components that are used by the immune system to defend the body (OpenStax Figure 21.2).

One may recall that the lymphatic system consists of lymphatic vessels (that transport lymph fluid), as well as lymph tissues (tonsils, Bronchus Associated Lymphoid Tissue, BALT and Mucosa Associated Lymphoid Tissue, MALT) and lymph organs (bone marrow, thymus, lymph nodes and spleen), which house lymphocytes (immune system cells) as well as other important cells and extracellular proteins that provide structure and function.  Most lymph organs have capsules made of fibrous connective tissue (e.g. lymph nodes, thymus, and spleen).  Lymph Tissues (e.g. tonsils and MALT) tend to be aggregates or clusters of lymphocytes within other organs or structures.  GALT (Gut-Associated Lymphoid Tissue) and Peyer’s patches are components of MALT which helps to protect the intestines from pathogens.

Figure 21.2 Anatomy of the Lymphatic System  Lymphatic vessels in the arms and legs convey lymph to the larger lymphatic vessels in the torso (OpenStax).

Lymph vessels are a pale golden colour and penetrate most tissues of the body intertwining with the capillaries therein.  Their structure and role is quite different than blood vessels though, in that lymphatic vessels begin with blind-ends (or closed ends) and serve to recapture interstitial fluid (within the interstitial spaces) that is continuously leaked from capillaries (Figure 21.3).  In total, approximately 3-4L is collected per day.  To readily perform this function, the endothelial cells of a lymphatic capillary are not bound tightly and overlap, acting as one-way valves which permit the entrance only of fluid, solutes, proteins, viruses, bacteria, and cell debris.  Once absorbed through these flap-like valves in the blind-ends of lymphatic capillaries, this fluid, now termed lymph fluid is strategically transported through lymph nodes before finally arriving at either the right or left subclavian vein into which the fluid is returned to the bloodstream.

Figure 21.3 Lymphatic Capillaries Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary (OpenStax).  The walls of lymph vessels are similar to veins in that there are 3 tunics, an inner endothelial layer, a middle smooth muscle layer, and an outer layer of connective tissue.  However the basement membrane is incomplete and the endothelial cells are arranged as flap-valves.  The smooth muscle layer is innervated and consists of an inner circular layer and an outer longitudinal layer of smooth muscle cells.  The luminal diameter of lymphatic capillaries is larger than blood capillaries and their walls are thinner.  Lymphatic capillaries have a lower hydrostatic pressure and higher osmotic pressure than blood capillaries.

Areas of the body that do not include lymphatic vessels include: the central nervous system, teeth, cornea and bone marrow.  It is important to note that lymph is not pumped, as is blood by the heart, but is moved through lymphatic capillaries, into larger-diameter lymph vessels, through lymph nodes, and finally into lymphatic ducts through peristalsis.  The rhythmic contraction of the smooth muscle lining of lymphatic vessels achieves this peristalsis which is particularly necessary as lymph is moved upwards against gravity.  The flow rate is approximately 4-5mm/second.   Compression of neighbouring skeletal muscle around lymph vessels also assists in the movement of lymph, as does the process of breathing which continuously introduces periods of lower thoracic pressure during inspiration that facilitate the upward movement of lymph through this region.  This phenomenon of regular pressure changes within the thorax is often called the respiratory pump.  Additionally, semi-lunar valves are strategically located along the length of lymphatic vessels which prevent back-flow.  Lymph fluid from the right half of the thorax, right arm and right side of the head and neck are carried through lymph vessels that merge into the right lymphatic duct which drains into the right subclavian vein.  Lymphatic vessels from the rest of the body eventually merge into the thoracic duct which drains into the left subclavian vein (Fig. 21.4).

Figure 21.4 Major Trunks and Ducts of the Lymphatic System  The thoracic duct drains a much larger portion of the body than does the right lymphatic duct (OpenStax).

Lymph nodes are small bean-shaped organs that house immune cells, specifically B and T lymphocytes, in large quantity which function to screen this lymph fluid making sure to destroy any pathogens, neutralize any toxins, and launch a full immune response if required in order to protect the body.  There are 500-600 lymph nodes in the body and there are clusters of lymph nodes in the groin (Inguinal Lymph Nodes), armpits (Axillary Lymph Nodes), neck (Cervical Lymph Nodes), chest, and surrounding the intestines.   The 3 largest collection of lymph nodes are the Inguinal, Axillary, and Cervical.  Lymph nodes can become swollen during infection as they trap incoming lymph while pathogens are destroyed.