Opsonizing antibodies against pneumococci detected by enhanced NBT. Reduction of phagocytizing granulocytes.

Specific opsonizing antibodies against pneumococci were demonstrated in serum by measuring their effect on the nitroblue tetrazolium (NBT) reduction of normal human granulocytes. In comparison to normal serum, convalescent sera from patients with severe pneumococcal disease caused a significant enhancement in NBT reduction in the presence of homologous antigen. Enhanced NBT reduction was observed in 24 of the 26 sera tested and was further increased by complement. The reaction was mainly type-specific, although a few cross-reactions did occur. A dose-response effect was observed when sera were tested in two-fold dilutions. The enhancement in NBT reduction could be partially eliminated by the absorption of sera with homologous antigens. The appearance and subsequent increase in opsonizing antibodies was observed in consecutive samples from one patient. This method can be useful in the study of immunological responses to pneumococcal vaccine or disease, since it measures antibodies which are essential for immune defence.