A study of C3b deposition on yeast surfaces by sera of known opsonic potential.

C3c fragments released by the actin of trypsin from C3b molecules bound to zymosan may be readily quantitated by conventional gel techniques to give a measurement of the efficiency of C3b opsonization. Using this approach, the rate of deposition of C3b molecules was found to be very rapid in sera known to opsonize yeast normally. In contrast, sera defective in yeast opsonization deposited C3b much more slowly. The C3b elution technique was found to correlate well with both a direct phagocytosis assay using baker's yeast (r = 0.87, P less than 0.001) and a recently described neutrophil iodide uptake assay (r = 0.88, P less than 0.001).