The Initiating Factor of the Alternative Pathway and the Assembly of the Properdin (P) Receptor Forming Enzyme

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Abstract
A trace protein of normal human serum (NHS) has been detected by its reaction with antisera to nephritic factor (NF). It appears to be an essential early acting component of the alternative pathway. C4-depleted NHS adsorbed with solid phase anti-NF failed to sustain C3 consumption upon incubation with inulin or zymosan (Z) despite undiminished levels of P, B, and C3. Addition of NF, P̄, or cobra venom factor, however, caused consumption of C3 in the adsorbed serum. Addition of the glycine-HCl eluate (pH 2.2) of the immune adsorbent reconstituted the depleted serum. When C4 and P depleted serum (NHS s̄ C4,P) was incubated with Z for 60 min at 37°, 15 to 25% C3 consumption was consistently observed. The low magnitude C3 consumption was abolished by prior treatment of NHS s̄ C4,P with solid anti-NF. By use of various immunochemically depleted sera and fluoresceinated antisera to C3 and P̄, it was found that C3 could be deposited on Z from NHS s̄ C4,P, but not from serum depleted of B, D̄, or the NF analogue.

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