Q. 39 The type-II hypersensitivity reaction is mainly mediated by ____ .
(A) IgE
(B) IgM
(C) IgA
(D) T cells
Type II hypersensitivity reactions involve antibody-mediated cytotoxicity targeting cell surfaces or extracellular matrix. The correct answer to the query is (B) IgM, as these reactions primarily rely on IgM and IgG antibodies.
Correct Answer
B. IgM
Type II hypersensitivity, also known as cytotoxic hypersensitivity, occurs when IgM or IgG antibodies bind to antigens on host cells, triggering complement activation, phagocytosis, or antibody-dependent cellular cytotoxicity (ADCC). This leads to cell lysis or dysfunction, as seen in conditions like autoimmune hemolytic anemia and transfusion reactions.
Option Analysis
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(A) IgE: Incorrect. IgE mediates type I hypersensitivity, causing immediate allergic reactions via mast cell degranulation, such as in asthma or anaphylaxis.
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(B) IgM: Correct. IgM, along with IgG, binds cell-surface antigens, activating complement pathways that result in opsonization, inflammation, and tissue damage.
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(C) IgA: Incorrect. IgA primarily protects mucosal surfaces and does not play a major role in type II reactions, which require complement-fixing antibodies like IgM.
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(D) T cells: Incorrect. T cells drive type IV delayed hypersensitivity, involving cell-mediated immunity rather than antibodies.
