Complement factor B Bb fragment (Bb)

The Bb fragment of complement factor B is a 60 kDa C-terminal product generated during activation of the complement alternative pathway, when factor D cleaves factor B bound to C3b or C3(H2O). Bb contains the serine protease domain of factor B and is responsible for the catalytic activity of both the C3 convertase (C3bBb) and the C5 convertase (C3b(n)BbC3b). The active convertase cleaves C3 and C5, amplifying the downstream immune and inflammatory responses. Free Bb fragment found in plasma is typically inactive but serves as a biomarker of complement activation. Bb participates in immune cell modulation, such as enhancing B cell proliferation under certain conditions. Dysregulation or excessive formation of Bb and the convertase complexes is associated with diseases of complement activation, including autoimmune, inflammatory, and infectious diseases. Therapeutic inhibition of factor B or Bb is of growing interest for complement-driven disorders, but care is needed due to the essential role of complement in host defense.

Other names

Bb fragment of complement factor BFactor B Bb fragmentComplement factor B serine protease domainBf BbCFB-Bb

Inhibition of Bb serine protease activity (by blocking formation or activity of the C3/C5 convertases) Prevention of convertase formation or promotion of convertase dissociation, reducing downstream complement activation