Receptors for IgM on human lymphocytes. I. Detection of receptors on freshly drawn lymphocytes and at physiological temperature.

Receptors for the Fc portion of IgM (RFcmu) have previously been found to be primarily associated with the T lymphocyte subpopulation in humans, and their detection has frequently required overnight incubation of the cells before assay. In the present study, using highly sensitized indicator cells (EAmu), lymphocytes with receptor for IgM (EAmu-RFC) were detected immediately after isolation from human peripheral blood. Identification of EAmu-RFC on freshly isolated lymphocytes was greatly facilitated by assay at room temperature (RT) or 37 degrees C, implying that analogous interactions between IgM-antigen complexes and lymphocytes with receptor for IgM normally occur in vivo. These results seem to indicate that some RFcmu on lymphocytes may be occupied by IgM and/or IgM-antigen complexes in vivo and that immediately after isolation many RFcmu are still occupied and unavailable for interaction with EAmu. It is suggested that the successful competition of IgM-antigen complexes for RFcmu on T lymphocytes is the signal for and/or mechanism of expression of help by the lymphocyte. The sensitivity of the assay for EAmu-RFC also permitted the identification of a small subpopulation of RFcmu+ lymphocytes in the non-T cell fraction of the peripheral blood of some individuals.