Opsonizing antibodies in human typhoid fever.

Factors that promote phagocytosis of Salmonella typhi by polymorphonuclear leukocytes have been detected in sera from patients with typhoid fever. These factors were present between the 10th and the 60th days after the clinical onset. They could be fixed on the bacteria, were heat-stable, and needed the classical pathway of complement to act. They were specific for Salmonella typhi and acted as antibodies directed against the O antigen of the cell-wall. They could be absorbed by a smooth strain of Salmonella typhi, but not by a rough strain. They belonged probably to the IgG class and were different from the "natural antibodies" of normal, non-immune serum, which are of IgM type. Their role in human resistance to Salmonella infection is discussed.