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]
Complement components and ligands in the Guide to Pharmacology. Family956, human C5a ligand entry and Comment
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]
Isolation of beta 1F-globulin from human serum and its characterization as the fifth component of complement by U. R. Nilsson and others. Primary Abstract: isolation, component assignment, kinetic ordering and C6 dependence
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]
Chemotactic and anaphylatoxic fragment cleaved from the fifth component of guinea pig complement by H. S. Shin and others. Abstract: separated C5 fragments and biological assays
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