Evaluation of a hemolytic assay of the alternative complement pathway in human serum

Alternative pathway activity of human serum was titrated by use of unsensitized rabbit erythrocytes (RE). Under the conditions of the assay, the von Krogh equation could be used to relate the proportion of RE lysed to the level of alternative pathway activity. The use of a 50% hemolytic endpoint provided maximum sensitivity in the assay. The 50% hemolytic endpoint could be calculated from a single measurement in the region of 20% to 80% lysis or RE. Factor B was required for lysis of RE in the test, but neither C2 nor C8 was limiting under the conditions of the assay. Alternative pathway activities of three sera with abnormal IgG levels were in the normal range, but normal serum absorbed with RE at 0 C before testing had diminished lytic activity with the test. Lysis of RE in acute-phase sera of 16 patients who had bacteremic pneumococcal pneumonia was significantly below normal (P < 0.01). Results with Re lysis in these patients correlated well with levels of Factor B that were measured immunochemically and with consumption of whole complement by zymosan.