C3/FB-Depleted Serum is normal human serum in which C3 and factor B (fB) have been removed by immunoaffinity chromatography. The product is tested for the absence of C3 and factor B activity by testing classical and alternative pathway function and for the absence of C3 and factor B proteins by double immunodiffusion. The C3/FB-depleted serum is certified to exhibit less than 5% classical pathway and alternative pathway activities. A functional classical pathway can be reconstituted by addition of purified C3 protein (1.3 mg/mL) whereas a functional alternative pathway can be reconstituted by addition of purified C3 protein (1.3 mg/mL) and factor B protein (0.2 mg/mL) indicating that all other complement components necessary for classical and alternative pathway activation are present. After reconstitution with C3 (1.3 mg C3/mL) the C3/FB-Depleted Serum is certified to possess a functional classical pathway for complement activation whereas after reconstitution with C3 (1.3 mg C3/mL) and factor B (0.2 mg/mL), the product is certified to possess a functional alternative pathway for complement activation (Morgan, B.P. (2000); Dodds, A.W. and Sim, R.B. (1997)). Although a functional lectin pathway should also be reconstituted by the addition of similar concentrations of C3, the function of this pathway is not tested.
Note that there is a weak C3 by-pass mechanism whereby the convertase C4b,C2a is able to activate C5 (Rawal, N. & Pangburn, M.K. 2003) and form C5b-9 complexes capable of causing low grade lysis. This system of C5 activation is functional in both the classical and the lectin pathways and is approximately 1000-fold less active than in the presence of C3.
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