Normal human serum was depleted of factor P (properdin) by immunoaffinity chromatography. The product is tested for the absence of factor P by double immunodiffusion. Factor P is a positive regulator of alternative pathway complement activation. As a result, Factor P-Dpl is still capable of activating the alternative pathway but activation is slower than with properdin present. For example, lysis of rabbit erythrocytes by Factor P-Dpl requires 20-25 min to reach 50% without properdin, but with P added back or in NHS lysis requires only 5-7 min under otherwise identical conditions. Factor P-Dpl is certified to possess a functional alternative pathway for complement activation which is fully functional if reconstituted with properdin. Full reconstitution requires addition of 10 µg factor P/mL serum (Law, S.K.A. and Reid, K.B.M. (1995)). The normal human serum concentration of factor P has been variously reported to be between 4 and 20 µg/mL (Morley, B.J. and Walport, M.J. (2000); Morgan, B.P. (2000); Dodds, A.W. and Sim, R.B. (1997)).
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