AbstractWe report the consequences of low expression of the T cell receptor (TcR)/CD3 complex by T lymphocytes from a 4‐year‐old boy with a mild immunodeficiency. TcR/CD3 expression was found to be deficient on both resting and activated T cells, using both anti‐CD3 and anti‐TcR α/β monoclonal antibodies. As shown by immunofluorescence and immunoprecipitation studies, residual expression (corresponding to about 10% of normal) was detectable on resting and activated TcR α/β+T cells. Other T cell membrane receptors were normally expressed. The functional consequences of this TcR/CD3 expression deficiency included an absence of T cell proliferation, interleukin 2 receptor expression and calcium flux following anti‐CD3 and anti‐CD2 antibody‐triggered T cell activation. Antigen (tetanus toxoid,Candidaand allogeneic cell)‐induced proliferation was detectable. In contrast, cytotoxic T cell activity towards allogeneic cells was deficient. These findings shed light on the function of the TcR/CD3 complex and indicate that the expression of a limited number of TcR/CD3 receptors may be sufficient to trigger antigen‐specific T cell activation (and, possibly, differentiation) and that anti‐CD3 antibody‐induced T cell activation differs somewhat from antigen/major histocompatibility complex molecule‐induced activation. These results also confirm that the CD2 pathway of T cell
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