The alternative pathway of complement is a host defence system that assists in maintaining the integrity of an organism by inactivating invading organisms, pathogens and modified tissue cells.
Complement component factor B is a single-chain, 93-kDa polypeptide. Its association with C3b makes the protein susceptible for proteolytic cleavage by factor D. The smaller cleavage product, the released Ba fragment (33 kDa), is represented by three short consensus repeat (SCR) domains, which are each encoded by a separate exon. The larger Bb fragment (60 kDa) is encoded by 13 exons and contains domains with homology to the catalytic chains of other serine proteases. Each of the functionally relevant domains of the active site is contained on a separate exon. Complement component factor B of the alternative complement pathway shows structural and functional homology to the classical pathway regulator C2.
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