C5a

C5a is a small peptide fragment derived from complement C5 and classified as an anaphylatoxin. It can act as a signaling ligand; the name identifies a processed product rather than the whole circulating precursor. The originating pharmacology record identifies human C5a as an endogenous ligand associated with inflammatory signaling.^[ref]

Historical experiments tied fragment isolation to biological activity. In 1968, researchers separated larger and smaller fractions from guinea pig complement C5. The smaller fraction attracted rabbit polymorphonuclear leukocytes and showed activity in a guinea pig ileum assay. Those observations concern specific assay systems, not a human clinical experiment.^[ref]

For an original interpretation example, suppose equal amounts of two preparations are compared in both a chemotaxis assay and a smooth muscle response assay. Similar movement of leukocytes would not by itself establish equal smooth-muscle effects: the two readouts measure different responses. Conversely, detecting a fragment by an antibody does not guarantee that it retains either biological activity.

This separation of identity, amount and function is essential when discussing C5a. An anaphylatoxin classification names a biological activity; it does not itself diagnose a clinical episode of anaphylaxis.