When monolayers of bovine erythrocytes (Eb) were exposed to purified human blood lymphocytes and either IgG or IgM fractions of rabbit anti-Eb serum, clear zones (plaques) appeared when Eb had been lysed by antibody-dependent effector cells (K cells). IgG-dependent plaque formation was complete by 20 h of incubation, while the IgM-dependent reaction required 40 h. The estimated minimal numbers of plaque forming cells (PFC) were 5.6% (IgG) and 2.0% (IgM) of the added lymphocytes. Inhibition experiments with human IgG or IgM indicated that different immunoglobulin receptors on the effector cells were involved in the two systems. In the IgG system, approximately 50% of the PFC had complement receptors and approximately 30% receptors for Helix pomatia A hemagglutinin (HP). In the IgM system, less than 10% of the PFC had complement receptors, while approximately 60% had HP receptors. The results suggest that a subset of human T cells had IgM-dependent K-cell potential. These cells are different from the majority of the IgG-dependent K cells.

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