Medical Definition of anaphylatoxin
: any of several peptides that promote acute inflammation and mediation of the immune response following activation of complement In the second type of [anaphylactic] reaction, immune complexes or other agents activate the complement cascade, resulting in the formation of anaphylatoxins (e.g., the complement protein fragment C3a and C5a), which can trigger the release of mediators from mast cells and basophils directly.—Bruce S. Bochner, The New England Journal of Medicine, 20 June 1991
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