Functional Relationship of Factor B in the Properdin System to C3 Proactivator of Human Serum

Summary Factor B in the properdin system is a heatlabile protein requiring, in addition to properdin, other serum factors, and Mg2+, for formation of a complex with zymosan which preferentially inactivates C3 and later acting components. Factor B is a β-pseudoglobulin which is distinct from C2. Evidence has been presented that factor B is functionally identical to an independently isolated serum protein, C3 proactivator (C3PA). These observations serve both to identify and characterize factor B and, in conjunction with earlier evidence, to establish the properdin system as an alternate pathway to C3 and later acting components, independent of C1, C4 and C2.