Lymphocytes are now known to be a heterogeneous cell population which contains at least two functional subtypes. One subtype of cell (B-lymphocyte) bears easily detectable amounts of membrane-associated immunoglobulin; studies in mice have established that these lymphocytes are of bone marrow origin and include the precursors of antibody-forming cells (1, 2, 3). A small number of cells within this population of surface immunoglobulin bearing lymphocytes from non-immunized animals specifically bind antigen and the frequency of such cells increases subsequent to immunization (4, 5). The other major subtype lacks easily detectable surface immunoglobulin and mediates a variety of cellular immune phenomena (6, 7). These lymphocytes in the mouse are derived from the thymus and possess surface differentiation antigens, such as θ, which allow their identification (1). Further delineation of the structure and function of lymphocyte subpopulation and of the interactions between them would be aided by development of techniques to obtain relatively pure populations.
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