Overview of general mechanism: Antibodies directed against target antigens on cells or in extracellular matrix. The target antigens may be endogenous or absorbed exogenous antigens.

Specific mechanisms: There are three specific mechanisms by which type II hypersensitivity reactions occur. The three mechanisms are complement-dependent reactions, antibodydependent cell-mediated cytotoxicity, and antibody-mediated cellular dysfunction.

Complement-dependent reactions
~ Mechanism: Antibody bound to antigen can fix complement and cause direct lysis of the cell through production of the membrane attack complex (MAC), or the complement can coat cells with C3b (an opsonin) and promote phagocytosis of the antigen.
~ Example: Glomerulonephritis.
■ Antibody-dependent cell-mediated cytotoxicity
~ Mechanism: Cell types that bear receptors for the Fc portion of IgG, such as neutrophils, eosinophils, macrophages, and natural-killer (NK) cells, mediate removal of antigen.
~ Examples: Transfusion reactions, erythroblastosis fetalis, and autoimmune hemolytic anemia.
■ Antibody-mediated cellular dysfunction
~ Mechanism: Antibodies themselves affect function of the antigen.
~ Examples: Graves disease is due to an antibody that activates the thyroid-stimulating hormone (TSH) receptor, resulting in hyperthyroidism. Myasthenia gravis is due to antibodies against the acetylcholine (ACh) receptor, impairing neuromuscular transmission.

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