Type II Hypersensitivity-Antibody Mediated
Type II Hypersensitivity is an antibody mediated process in which IgG and IgM antibodies are directed against antigens on cells or extracellular material. This subsequently leads to cell lysis, tissue damage or loss of function.
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Antibodies – IgG and IgM antibodies
Target antigens
Intrinsic- The antigens may be normal molecules intrinsic to cell membranes or in the extracellular matrix.
Extrinsic- They may be adsorbed exogenous antigens (e.g., a drug metabolite such as penicillin)
Antibody-Mediated cytotoxic reactions (type II Hypersensitivity) mechanism-When antibody binds to an antigen on the surface of a target cell, it can cause damage through following mechanisms.
Opsonization and phagocytosis- When cells are coated with autoantibodies, with or without complement proteins, the cells become targets for phagocytosis. This coating with autoantibodies is opsonization, and neutrophils and macrophages phagocytose them. Phagocytes express receptors for the Fc tails of IgG antibodies coating these cells and for breakdown products of the C3 complement protein.
Examples
- Transfusion reactions
- Hemolytic disease of the newborn (erythroblastosis fetalis)
- Autoimmune hemolytic anemia, agranulocytosis, and thrombocytopenia
- Drug reactions
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Inflammation-Antibodies bound to cellular or tissue antigens activate the complement system by the classical pathway. Activated complements recruit neutrophils and monocytes, triggering inflammation in tissues, which is responsible for tissue injury.
Example
- Glomerulonephritis (few types)
- Goodpasture syndrome
- Vascular rejection in organ grafts
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Antibody-mediated cellular dysfunction- Antibodies directed against a host protein impair or dysregulate important functions for example in Graves disease, antibodies against the thyroid-stimulating hormone receptor stimulate thyroid epithelial cells to secrete thyroid hormone in excess leading to hyperthyroidism.
Example
- Graves disease
- Myasthenia gravis
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Case Scenarios
A 13-year old boy is diagnosed with infection by a helminthic parasite. Eosinophils require which antibody isotype to destroy these parasites via antibody-dependent cellular cytotoxicity?
A 36-year-old woman presents to her primary care physician for fatigue. Her fatigue is worse towards the end of the day. At times when she is watching television or reading a book she noticed double things. She noticed right eyelid droop recently. On physical exam, there is right-sided ptosis and muscle weakness. Serologic studies return positive for anti-acetylcholine receptor antibodies. Which type of Hypersensitivity is associated with her illness??
Further reading https://www.onlinebiologynotes.com/type-ii-hypersensitivity-reaction-mechanism-and-examples/
Revision for today https://creativemeddoses.com/topics-list/babesiosis/