Studies on Heat-Labile Opsonin in Rabbit Serum

Abstract

A technique has been developed which allows reliable quantitative assessment of phagocytosis of bacteria by rabbit granulocytes in vitro, and which permits study of the role of heat-labile serum factors in the phagocytic process. Incubation of bacteria with fresh serum opsonized the organisms. After washing they were fully susceptible to ingestion in a serum-free system. Using this preincubation technique the influence of ionic composition of the medium was observed on the opsonization and on the ingestion processes separately. High concentration of salt blocked both processes. Divalent cations were required for ingestion but not for opsonization of the bacteria. The lack of requirement for calcium and magnesium speaks against identification of heat-labile serum opsonin with the complete hemolytic complement system. Features of heat-labile serum opsonin which distinguished it from antibody were the following: a) heat-lability, b) titer for various bacteria remarkably constant in various serum specimens, c) rapid reaction with bacteria at 38°C but not at 0°C, d) destruction by hydrazine or ammonia, e) not γ globulin, and f) lack of specificity as demonstrated in adsorption studies.