= Anaphylatoxin
An \b[anaphylatoxin] is a small signaling <peptide> released from a <complement system> component during activation. <C3a> and <C5a> are important examples: fragments derived from larger circulating <proteins> act as <ligands> for <cell-surface receptors>, connecting <complement system> activation with cellular responses involved in <inflammation>.<Complement components and ligands in the Guide to Pharmacology>[IUPHAR/BPS ligand-family record: human C3a, ligand 3640, and C5a, ligand 573; C5a receptor association in the C5a ligand record.]{ref}
The historical name is best understood through measurements of activity. <Viktor A. Bokisch> and colleagues isolated <C3a> and measured its ability to contract <smooth muscle>. Its activity could be inhibited or lost without simply equating every <Complement C3> fragment with the same effect. This supplies a concrete example of a <protein> cleavage product acquiring a signaling role distinguishable from the remainder of its precursor.<Isolation of a fragment (C3a) of the third component of human complement containing anaphylatoxin and chemotactic activity and description of an anaphylatoxin inactivator of human serum by Viktor A. Bokisch and others>[Abstract: isolated C3a, smooth-muscle activity, antibody inhibition and inactivation.]{ref}
An <anaphylatoxin> is therefore a host-derived signal, rather than necessarily an externally introduced poison. A response to a fragment must be specified by the <receptor>, responding <cell> and experimental readout.
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