C3a is a small protein fragment released when Complement C3 is cleaved during activation of the complement system. It is an anaphylatoxin, associated with inflammatory responses, and is distinct from the larger C3b product involved in surface-associated complement reactions. Proteolysis can thus produce different activities from different parts of one precursor.[ref]
Structures of complement component C3 provide insights into the function and evolution of immunity by Bert J. C. Janssen and others. Introduction and Figure1: C3 activation and fragment organization
In 1969, Viktor A. Bokisch and collaborators isolated C3a by several methods and tested its smooth muscle-contracting activity. Antiserum directed against C3a inhibited the biological response, connecting the isolated fragment to the activity rather than identifying it solely by a separation band. Further enzyme treatment changed its measured activities, showing why the amount of a fragment and its retained function are separate questions.[ref]
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: isolation, smooth-muscle response, antiserum inhibition and enzyme treatment
An idealized bookkeeping example starts with 50 molecules of Complement C3, each undergoing exactly one activation cleavage. It produces 50 C3a fragments and 50 C3b fragments: 100 product molecules, but no doubling of total protein material. In an actual sample, production, further processing and removal all affect the measured concentration of C3a.
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