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An 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.^[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.^[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.

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.

C5b is the larger protein fragment produced by cleavage of complement C5. It initiates assembly of the terminal complement system components that form the membrane attack complex. Its companion cleavage product C5a has a different signaling role. The unprocessed precursor, C5a and C5b should therefore be treated as distinct molecular species.^[ref]

The initial assembly recruits complement C6 and further components. C5b supplies an initiating platform rather than constituting a complete membrane pore by itself. In a 2012 structural investigation, the measured full-length complement C6 structure supported a proposed mechanism for sequential assembly around that platform. The paper's model and its directly determined crystal structure are different kinds of evidence.^[ref]

For original molecular accounting, cleavage of ten intact complement C5 molecules can yield ten C5b fragments and ten C5a fragments if every cleavage is successful and the products are retained. That does not create twenty complement C5 molecules: the fragments divide the precursor's material. An assay recognizing a sequence shared by the precursor and C5b would need an additional way to distinguish processing states.

Likewise, detecting ten C5b fragments does not demonstrate ten finished membrane attack complexes. Recruitment of partners and productive assembly remain necessary. Measuring the amount of a fragment, formation of a complex and a functional endpoint such as hemolysis addresses three related but distinct questions.

Complement C5 is a blood serum protein participating in the complement system. Its intact precursor and its released fragment C5a are different molecular species with different activities. Detecting the precursor is therefore not equivalent to measuring the response elicited by a cleavage product.^[ref]

A 1965 experiment isolated the serum protein called beta1F-globulin and assigned it to the fifth complement component. Kinetic comparisons placed its reaction after Complement C3, and its reported hemolysis function depended on complement C6. Separating components and testing their dependence supplied functional evidence about reaction order, beyond merely detecting a serum band.^[ref]

For an original assay-design comparison, measure both the amount of complement C5 in a preparation and the activity recovered when it is combined with the other defined components. Equal protein amounts could give different functional responses if processing or the availability of a required partner differs. A signal reporting protein abundance alone cannot identify which step failed.

A later fragment study adds a second distinction: the small active fraction obtained from guinea pig complement C5 produced measured chemotaxis and smooth muscle responses. The behavior of a released fraction should not be silently assigned to the entire unprocessed protein.^[ref]