C5b7

C5b7 is a transient intermediate in the terminal pathway of the complement system, involved in assembling the membrane attack complex (MAC). It forms during the conversion of C5 to C5a and C5b, when the C5b fragment binds to the next component in the cascade, C7, producing the C5b-7 complex. In many descriptions, this step follows C5b binding to C6 to form C5b6, which then binds C7 to yield C5b-7.

The C5b-7 complex is amphipathic and readily associates with lipid membranes, but it remains unstable in aqueous solution and is typically membrane-anchored rather than freely circulating. Binding of subsequent components proceeds with C8 binding to C5b-7 to form C5b-8, followed by polymerization and insertion of C9 to generate the final membrane attack complex, C5b-9, which forms a pore in the target cell membrane and can lead to lysis.

Physiological and clinical relevance: MAC formation is a critical defense mechanism against certain pathogens, particularly Neisseria species. Deficiencies in components of the MAC (C5–C9) or regulatory abnormalities can increase susceptibility to infections. Conversely, excessive MAC activity or dysregulated activation can contribute to tissue injury in inflammatory or autoimmune contexts. The presence and activity of C5b-7 reflect upstream complement activation and are related to the overall capacity of the terminal pathway to form MAC.

Terminology: C5b7 denotes the C5b fragment bound to C7, an essential early stage in MAC assembly named for the components involved.