Depression of natural killer cell activity by syphilitic serum and immune complexes.

Consecutive serum samples were obtained from patients with syphilis before and on three occasions after treatment. The sera contained immunosuppresive factors associated with the immunoglobulin fraction, which could depress the natural killer cell activity of healthy controls. There was no evidence that allogeneic or lymphocytotoxic antibodies played a role in immuno-suppression, which could be reproduced with both soluble and insoluble antigen-IgG-antibody complexes.