C5b

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.