1. A substance postulated to be the immediate cause of anaphylactic shock and that is assumed to result from the in vivo combination of specific antibody and the specific sensitizing material, when the latter is injected as a shock dose in a sensitized animal.
2. The small fragment (C3a) split from the third component (C3) of complement by C3 convertase and that releases histamine from rat peritoneal mast cells, causes pig ileum to contract, and produces a local wheal following intracutaneous injection in man; also used with reference to a small fragment (C5a) split from the fifth component (C5) of complement by the EAC1243 complex which has chemotactic properties as well. Syn: anaphylotoxin.
Origin
[anaphylaxis + toxin]