Complement factor B fragment Bb (Bb)

Complement factor B fragment Bb is the catalytically active, C-terminal serine protease fragment generated when circulating complement factor B is cleaved by factor D during activation of the alternative complement pathway. The Bb fragment binds to complement component C3b to form the C3bBb complex, which acts as the C3 convertase of the alternative pathway, catalyzing the cleavage of C3 to C3a and C3b and, with additional C3b, acts as the C5 convertase for C5 cleavage. This process enables rapid amplification of the complement response, driving pathogen opsonization, inflammation, and immune cell recruitment. Bb is central to several inflammatory, autoimmune, and complement-mediated diseases, making it a validated therapeutic target for new drugs designed to selectively block the alternative pathway and reduce complement-mediated tissue damage without fully disabling host defense. Bb itself is a biomarker for alternative complement pathway activation and disease activity in complement-mediated disorders.

Other names

BbFactor B Bb fragmentCatalytic fragment of complement factor BComplement factor B Bb subunit

Inhibition of Bb serine protease activity to block alternative pathway C3/C5 convertase formation; Reduction of C3a, C3b, C5a, C5b generation; Diminished terminal complement pathway and membrane attack complex (MAC) formation