109 Immune Disorders – Type II Hypersensitivity Reactions

Type II Cytotoxic Hypersensitivity Reactions are antibody-mediated reactions involving antibodies binding to and targeting cells or extracellular material for destruction.

In the first example, extracellular material, such as the basement membrane of the lungs and kidneys are inappropriately targeted by an individual’s own antibodies in the rare auto-immune condition, Goodpasture Syndrome.

In the next examples, antibodies inappropriately target cells for destruction through three main mechanisms.

The first mechanism is antibody-dependent cellular cytotoxicity (ADCC, also known as antibody dependent cell-mediated cytotoxicity), where antibodies act as opsonins without requiring complement proteins, leading to targeted cell destruction by white blood cells (WBCs). For instance, Natural Killer cells bind to attached IgG antibodies and release perforin and granzyme to cause cell lysis or apoptosis.  The antibody opsonins also facilitate phagocytosis by neutrophils, macrophages, and eosinophils, which attach to the IgG (or IgE) antibodies to engulf the targeted cells. This mechanism is seen in the rare autoimmune disease, Autoimmune Hemolytic Anemia (AIHA).

The second mechanism involves antibodies binding to targeted host cells and facilitating classic complement system activation, leading to cell lysis, as occurs in incompatible blood transfusion which result in Acute Hemolytic Transfusion Reactions (AHTR).

The third mechanism involves the antibodies inappropriately disrupting or changing the function of targeted host cells, as seen in the autoimmune diseases Myasthenia Gravis (MG) and Graves Disease (GD). In MG, auto-antibodies bind to acetylcholine receptors (AChRs) on the motor end plates in neuromuscular junctions, preventing acetylcholine from binding and activating these AChRs, resulting in flaccid paralysis, usually affecting the facial and eye muscles initially, but potentially progressively affecting muscles throughout the body. In GD, auto-antibodies bind to Thyroid Stimulating Hormone (TSH) receptors on thyrocytes, stimulating excess thyroid hormone production, leading to hyperthyroidism.

The second mechanism depicts the most common example of a Type II Hypersensitivity Reaction, which is Acute Hemolytic Transfusion Reaction (AHTR).  For example, let’s examine the case in which a person with Type A blood mistakenly receives Type B blood. As a quick reminder, the recipient’s Type A blood has A antigens on their RBCs, while the donor’s Type B blood has B antigens.

• Step One: The recipient’s blood plasma contains anti-B IgM antibodies that bind to the B antigens on the donated RBCs, with anti-B IgG antibodies also involved to a lesser extent.
• Step Two: Antibody binding to donated RBCs facilitates complement system activation, leading to cell destruction through three mechanisms.
  • o First, complement proteins form membrane attack complexes (MACs), creating portals in the donated RBCs, making them leaky, swell, and undergo osmotic lysis (cytolysis).
  • o Second, both complement proteins and antibodies act as opsonins, enhancing phagocytosis of donated RBCs by phagocytes like neutrophils, macrophages, and dendritic cells.
  • o Third, activated complement proteins enhance mast cell activity, releasing pro-inflammatory cytokines and chemokines that attract and recruit WBCs and induce coagulative cascades.

Signs and symptoms of AHTR occur quickly, within 24 hours, and include fever, chills, pruritus, urticaria, nausea, vomiting, and wheezing due to the release of WBC cytokines. Additionally, WBC cytokines and RBC damage activate the coagulation cascade, causing disseminated intravascular coagulation (DIC), leading to blood clots and bleeding issues, which can cause ischemia, organ hypoxia, and failure, particularly in the kidneys.

Furthermore, the lysis of donated RBCs releases hemoglobin into the plasma, overwhelming the body’s ability to bind and recycle it, causing hemoglobin accumulation in renal vasculature, resulting in damage, ischemia, and necrosis. Hemoglobin leaks through damaged glomeruli into the filtrate, resulting in hemoglobinuria and red urine. Serum levels indicate hemoglobinemia and bilirubinemia, potentially leading to jaundice.

Initially, hypertension can occur due to hemoglobin scavenging most of the nitric oxide (NO), a potent vasodilator produced by blood vessel endothelial cells. Without NO, vasoconstriction occurs, causing hypertension and contributing to renal failure.

Pro-inflammatory cytokines can lead to systemic inflammation, massive vasodilation, and increased capillary permeability, resulting in low blood pressure (hypotension). Hypotensive shock causes anxiety, tachycardia, and can lead to organ failure. Pulmonary edema can result in respiratory distress, and hypoxia to the brain can manifest as decreased consciousness, anxiety, and tachycardia. Signs of kidney damage include hematuria, oliguria, and flank pain.

Treatment involves discontinuing the blood transfusion and initiating fluid replacement as well as addressing hypotension. AHTR most commonly occurs from ABO group incompatibility but can also occur due to antibodies reacting with other RBC surface antigens (e.g., Kell antigens), emphasizing the importance of cross-match testing before transfusion.

A second scenario involves Hemolytic Disease of the Newborn (HDN), also known as Erythroblastosis Fetalis, which most commonly occurs when a Rh-negative biological mother is pregnant with a Rh-positive fetus. Fetal blood may mix with maternal blood at birth or due to an accident before birth, termed a sensitizing event. After exposure to the Rh antigen on fetal RBCs in a sensitizing event, the mother’s immune system produces anti-Rh antibodies, potentially causing HDN.  Anti-Rh antibodies are IgG antibodies which are small enough to cross the placenta and affect the fetus.  For example, during the current pregnancy or in subsequent pregnancies with Rh-positive fetuses, maternal anti-Rh antibodies can cross the placental barrier and attack fetal RBCs in the fetus, resulting in hemolysis, fetal anemia, potential stillbirth, or jaundice (due to high bilirubin levels, also known as hyperbilirubinemia).  Additionally, neonates with HDN may also exhibit high reticulocyte counts, low neutrophil counts (neutropenia), and low thrombocyte counts (thrombocytopenia).

Prevention and treatment of HDN involve injecting Rh-negative biological mothers with RhoGAM (anti-Rh antibody) as specific time points during the pregnancy to prevent sensitization from occurring.  Specifically, RhoGAM can bind to any fetal Rh-positive RBCs that enter the mother’s circulation, targeting these RBCs for destruction before the mother’s immune response becomes sensitized, preventing the production of maternal anti-Rh antibodies that could cross the placenta and cause HDN.  RhoGAM is typically administered at time points during and after pregnancy.  Other preventive methods include plasmapheresis to remove anti-Rh antibodies from maternal blood, especially if the mother has already developed anti-Rh antibodies from previous pregnancies.  Treatment of neonates with HDN is symptom depending and usually involves phototherapy to convert unconjugated bilirubin to conjugated bilirubin, which is water-soluble and easier to excrete.  This is important as high bilirubin levels are dangerous and can lead to kernicterus.  Kernicterus is neurological damage due to toxic effects of hyperbilirubinemia that can particularly affect an infants’ developing brains if not treated.

Most anti-A and anti-B antibodies are IgM class antibodies and too large to cross the placenta. However, ABO HDN can occur, though usually with milder symptoms than Rhesus (Rh) HDN. ABO HDN can happen when the fetus is Type A, Type B, or Type AB, and the mother is Type O and has been sensitized to Type A and B antigens or similar antigens exhibiting molecular mimicry produced by microflora or present in other aspects of the environment.  Once sensitized, a small proportion of anti-A and anti-B antibodies produced can be IgG class, and pass through the placental barrier.  However, since these antibodies bind to many cell types (not just RBCs), and the fetal A and B antigens on RBCs are less developed, the extent of hemolysis is usually less than in Rh HDN, leading to milder symptoms, though anemia and jaundice can still occur.

Summary

• Type 2 Cytotoxic Hypersensitivity Reactions
  • Defined as an antibody-mediated reaction in which IgG and IgM antibodies bind to cells or extracellular material (e.g., for example, the basement membrane of the lungs and kidneys in the rare autoimmune disease Goodpasture syndrome)
  • In Type II Cytotoxic Hypersensitivity Reactions in which antibodies inappropriately target host cells for destruction or change of function, antibodies utilize three mechanisms for destruction or change of unction including:
    • 1)  antibody-dependent cellular cytotoxicity (ADCC, also known as antibody-dependent cell-mediated cytotoxicity) in which antibodies act as opsonins (without requiring complement proteins) and facilitate destruction of the cell by adhering to and stimulating activation of WBCs.  For example, Natural Killers cells bind to attached IgG and release perforin and granzyme to cause the cell to lyse or go through apoptosis. Powerful phagocytes (e.g., neutrophils, macrophages, and eosinophils) can also attach to IgG or IgE antibodies and kill the targeted cells.  This plays out in infected cells as part of the innate immune response, and also in Type II Cytotoxic Hypersensitivity Reactions such as Autoimmune Hemolytic Anemia.
    • 2)  antibodies bind to targeted host cell and facilitate classic complement system activation and ensuing cell lysis (e.g., this occurs in acute hemolytic transfusion reactions, AHTR)
    • 3) loss or change of function of targeted host cell, such as in the autoimmune diseases Myasthenia Gravis (MG) and Graves Disease (GD) respectively.  In MG, auto-antibodies are inappropriately produced and bind to acetylcholine receptors (AChRs) in the motor end plates of neuromuscular junctions preventing acetylcholine from binding and activating these AChRs, resulting in flaccid paralysis, usually within facial muscles, though can progress to involve muscles throughout the body.  In GD, autoantibodies are inappropriately produced that bind to Thyroid Stimulating Hormone (TSH) receptors on thyrocytes, stimulating the production of excess thyroid hormone, leading to hyperthyroidism.  See Autoimmune Disease section for more details.
  • Example of Type II Hypersensitivity Reactions – Acute Hemolytic Transfusion Reaction (AHTR) = Bad blood transfusion
  • Scenario: Person with Type A blood mistakenly receives Type B blood.
    • Recipient’s Type A blood has A antigens on the surface of RBCs.
    • Donor’s Type B blood has B antigens on the surface of RBCs.
    • Recipient’s blood plasma contains anti-B IgM antibodies that bind to the B antigens on the surface of donated Type B RBCs.  Anti-B IgG antibodies are also involved, though to a lesser extent.
    • Antibody binding to donated RBCs facilitates the binding of complement system plasma proteins and the cells are targeted for destruction by 3 mechanisms:
    • Firstly, the complement proteins form membrane attack complexes (MACs) which create portals in the donated RBCs, making them leaky, swell, and undergo osmotic lysis (cytolysis).  This is how AHTR received its name, as the rupture of blood cells is known as hemolytic.
    • Secondly, both complement proteins and antibodies act as opsonins, enhancing phagocytosis of donated RBCs by phagocytes (e.g., neutrophils, macrophages, dendritic cells)
    • Thirdly, activated complement proteins enhance mast cell activity which degranulate to release pro-inflammatory cytokines and chemokines that attract and recruit WBCs and induce coagulative cascades
    • Signs and Symptoms:
      • occur quickly, within 24 hours = hence name acute hemolytic transfusion reaction
      • the release of WBC cytokines induce fever, chills, pruritus, urticaria, nausea, vomiting, wheezing.
      • WBC cytokines and RBC damage activates the coagulation (blood clotting) cascade causing disseminate intravascular coagulation (DIC) leads to blood clots and bleeding issues which can cause ischemia in various locations throughout the body, resulting in organ hypoxia and failure, particularly within the kidneys.
      • The lysis of RBCs release hemoglobin which overwhelm the body’s ability to bind and recycle this protein.  Hemoglobin accumulates in renal vasculature causing damage, ischemia and necrosis.  The leaking of hemoglobin through damaged glomeruli into the filtrate, results in hemoglobinuria and red urine.  Serum levels indicate hemoglobinemia, and bilirubinemia (can lead to jaundice).
      • Initially hypertension can occur, due to hemoglobin scavenging nitric oxide (NO, a potent vasodilator produced by blood vessel endothelial cells).  Without NO, vasoconstriction occurs, which can cause hypertension as well as contribute to renal failure.  Pro-inflammatory cytokines can lead to
      • Systemic Inflammation (and massive vasodilation and increased capillary permeability) results in low blood pressure (hypotension). Hypotensive shock causes the following symptoms: anxiety, tachycardia, and can lead to organ failure.
      • Pulmonary edema can result in respiratory distress
      • Hypoxia to the brain can manifest as decreased consciousness, anxiety, tachycardia.
      • Signs of kidney damage include hematuria, oliguria, and flank pain.
      • Treatment involves fluid replacement, and treatment of hypotension
    • Overview:  Acute Hemolytic Transfusion Reactions most commonly occurs from ABO group incompatibility, but can occur due to the presence of other antibodies (e.g., anti-Rhesus antibodies such as anti-D and anti-E, and anti-Kell) reacting with other RBC surface antigens (Rh and Kell).  This emphasizes the importance of cross-match testing prior to transfusion to ensure successful blood transfusion.
• Second Scenario:  Hemolytic Disease of the Newborn (HDN) also known as erythroblastosis fetalis:
  • Scenario: Most commonly occurs when a Rh-negative biological mother (birth parent) is pregnant with a Rh-positive fetus.
    • Fetal blood may mix with maternal blood at birth (or prior to birth if there is an accident and blood leaks past placental barrier – biological mother falls down stairs).  This event is termed a sensitizing event, in that after exposure to Rh antigen on RBCs of fetus, the biological mother’s immune system produces anti-Rh antibodies (IgG), leading to the possibility of HDN in future pregnancies.
    • In subsequent pregnancies, in which the fetus has Rh-positive RBCs, the maternal IgG antibodies cross the placenta and attack fetal RBCs -> resulting in hemolysis and fetal anemia, potential stillbirth or jaundice (due to high concentration of circulating bilirubin (hyperbilirubinemia), the breakdown product of hemoglobin).  Neonates with HDN may also exhibit high reticulocyte counts, and low counts of neutrophils (neutropenia), low counts of thrombocytes/platelets (thrombocytopenia)
• Prevention and Treatment:
  • Prevention:  biological mother with Rh-negative blood is injected with RhoGAM (anti-Rh antibody) to prevent sensitization.
    • Given to Rh-negative mothers before blood mixing.
    • RhoGAM will bind to any fetal Rh-positive RBCs that mix and enter the biological mother’s circulation.  RhoGAM antibodies will target these RBCs for destruction before biological mother’s immune response becomes sensitized, avoiding the production of maternal antibodies that would cross the placental barrier and cause HDN.
    • Typically administered during and after pregnancy.
  • Other Preventative Methods:  Plasmapheresis to physically remove antibodies from maternal blood.  This is helpful especially when a female has already developed anti-Rh antibodies (possibly from previous pregnancies).
  • Treatment of neonate with HDN:  Treatment of hyperbilirubinemia and jaundice involves phototherapy, to convert unconjugated bilirubin to conjugated bilirubin, which is water-soluble and easier to excrete.
• Additional Notes:
  • Most anti-A and anti-B antibodies are IgM, too large to cross the placenta.  However, ABO HDN can occur, though usually signs and symptoms are much milder than Rhesus HDN.  This can occur when the fetus is Type A, Type B or Type AB and biological mother is Type O and has been sensitized to Type A and Type B antigens or antigens that exhibit molecular mimicry to these antigens (thought to be produced by microflora).  A small proportion of anti-A and anti-B antibodies can be IgG and pass through the placental barrier.  However, as anti-A and anti-B antibodies will bind to many cell types (not just RBCs), additionally, the fetal A and B antigens are less developed which can reduce antibody binding and affinity.  Therefore, the extent of hemolysis in ABO HDN is usually less than Rh HDN, leading to milder symptoms, although anemia, and jaundice and still occur.
  • High levels of bilirubin are always dangerous as it can lead to kernicterus (neurological damage due to toxic effects of hyperbilirubinemia) particularly in the developing brains of infants, if not treated.
  • Anti-Rh antibodies are IgG, small enough to cross the placenta and affect the fetus.