Complement factor B Bb fragment

Factor B, part of the alternate pathway of complement activation, is cleaved by factor D into 2 fragments: subunits Ba and Bb. Bb, a 60 kDa serine protease, then combines with complement factor 3b (C3b) to generate the C3 or C5 convertase. The Bb fragment may become dissociated from the C3 or C5 convertases either through spontaneous decay or via the action of Factor H or CR. Since the Bb molecule is unique to the alternative pathway of complement, levels of Bb provide an excellent marker for complement activation via this pathway. It has also been implicated in proliferation and differentiation of preactivated B-lymphocytes, rapid spreading of peripheral blood monocytes, stimulation of lymphocyte blastogenesis and lysis of erythrocytes.

Swiss-Prot Accession Number: P00751