anaphylatoxin

noun

an·​a·​phyl·​a·​tox·​in ˌan-ə-ˌfil-ə-ˈtäk-sən 

: 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