LYMPHOCYTES are composed of two main populations: thymus-derived (T) lymphocytes which are largely responsible for cell-mediated immunity, and bursal-equivalent or bone-marrow derived (B) lymphocytes which are primarily involved in humoral immunity. B cells can be identified by the presence of fluorescence-detectable surface immunoglobulin1-3 and by their ability to bind antigen-antibody complexes or heat-aggregated immunoglobulin through the Fc portion of the immunoglobulin molecule4-7. Human T cells can be identified by their ability to form non-immune rosettes (E rosettes) with sheep red blood cells (SRBC)8-12. We report here that when these B and T cell markers were evaluated simultaneously oh individual cells, a small subpopulation reproducibly bore both markers. Further experiments seemed to rule out artefact as an explanation.
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