Journal of lmmunologicaI Methods, 28 (1979)105--116 105 © Elsevier/North-Holland Biomedical Press RECEPTORS FOR IgM ON HUMAN LYMPHOCYTES. I. DETECTION OF RECEPTORS ON FRESHLY DRAWN LYMPHOCYTES AND AT PHYSIOLOGICAL TEMPERATURE MICHAEL W. FANGER and PETER M. LYDYARD Department of Immunology, Middlesex Hospital Medical School, London W1, U.K. (Received 29 November 1978, accepted 18 January 1979) Receptors for the Fc portion of IgM (RFcp) have previously been found to be prima- rily 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 (EAp), lymphocytes with receptor for IgM (EAp- RFC) were detected immediately after isolation from human peripheral blood. Identifica- tion of EAp-RFC on freshly isolated lymphocytes was greatly facilitated by assay at room temperature (RT) or 37°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 RFcp on lymphocytes may be occupied by IgM and/or IgM- antigen complexes in vivo and that immediately after isolation many RFcp are still occu- pied and unavailable for interaction with EAp. It is suggested that the successful competi- tion of IgM-antigen complexes for RFc/l on T lymphocytes is the signal for and/or mech- anism of expression of help by the lymphocyte. The sensitivity of the assay for EAp-RFC also permitted the identification of a small subpopulation of RFcp ÷ lymphocytes in the non-T cell fraction of the peripheral blood of some individuals. INTRODUCTION Early studies in chickens, mice and rats have suggested that IgM can pas- sively absorb to lymphocytes both in vivo and in vitro (Webb and Cooper, 1973; Hudson et al., 1974; Hunt and Williams, 1974). More recently, recep- tors for the Fc portion of IgM (RFcp) have been detected primarily on a subpopulation of human T cells (Moretta et al., 1975; McConnel and Hurd, 1976). This subpopulation appears functionally and morphologically distinct from a minor T cell subpopulation carrying receptors for the Fc portion of IgG (RFc3,) (Moretta et al., 1977a; Grossi et al., 1978). Whereas T cells with RFcp appear to help B cells to differentiate into plasma cells following stim- ulation by Pokeweed mitogen (PWM), T cells bearing RFc3' suppress differ- entiation in the same assay (Moretta et al., 1977a). This functional distinction may not be absolute since under certain conditions cells bearing receptors for IgM can be induced to suppress (Hayward et al., 1978). The requirement