= Complement C5
<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.<Complement components and ligands in the Guide to Pharmacology>[Family956, human C5a ligand entry and Comment]{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.<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]{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>.<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]{ref}
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