Which of these antibodies mediates anaphylaxis?

Choice A rationale Immunoglobulin A (IgA) is the predominant antibody found in mucosal secretions like saliva, tears, breast milk, and gastrointestinal fluid, providing local mucosal immunity against ingested or inhaled pathogens. While vital for defense, IgA does not typically trigger the systemic mast cell degranulation and massive mediator release characteristic of anaphylaxis. Choice B rationale Immunoglobulin E (IgE) is the antibody that mediates Type I hypersensitivity reactions, including anaphylaxis. IgE is bound to the surface of mast cells and basophils. Upon re-exposure to a specific antigen (allergen), cross-linking of these IgE molecules triggers a massive, rapid release of pre-formed mediators, such as histamine and leukotrienes, causing the severe, systemic symptoms like profound vasodilation and bronchoconstriction. Choice C rationale Immunoglobulin G (IgG) is the most abundant antibody in plasma, providing long-term humoral immunity and crossing the placenta to confer passive immunity to a fetus. It is a primary mediator of Type II and Type III hypersensitivity reactions. Although it can activate the complement cascade, IgG does not play the principal role in the immediate, IgE-driven pathogenesis of anaphylaxis. Choice D rationale Immunoglobulin M (IgM) is the largest antibody, often existing as a pentamer, and is the first antibody produced during a primary immune response, primarily located in the blood and lymph fluid. It is highly effective at activating the complement system and agglutination but is not the specific key mediator responsible for initiating the rapid mast cell degranulation that defines anaphylaxis.