Complement Factor P

Complement factor P, better known as properdin, is the only physiological positive regulator of the complement system. Secreted mainly by neutrophils and other leukocytes as dimers and tetramers built from thrombospondin type 1 repeat (TSR) modules, properdin binds C3b and factor B within the nascent alternative-pathway convertase and dramatically extends its short half-life, amplifying complement activation on microbial surfaces. It can also initiate convertase formation independently of fluid-phase C3. X-linked properdin deficiency leaves patients highly vulnerable to fulminant meningococcal infection. Recombinant properdin is used in convertase-stabilization assays and innate-immunity research.