The marginal zone of the spleen contains lymphocytes with an intermediate morphologic form between small lymphocytes and plasmablasts. The majority of cells can be stained by B cell monoclonal antibodies including B1, Leu-14, and Leu-12. The most significant finding is the expression of an interleukin 2 receptor (Tac) and an enzyme-alkaline phosphatase by the marginal zone B lymphocytes. The Tac antigen is not normally present in B cells, but can be found in a portion of in vitro-activated B lymphocytes. In conjunction with other evidence, the marginal zone B lymphocytes may represent the activated B cells in the early stages of B cells differentiation. Alternatively, the Tac-positive, marginal zone B lymphocytes may be a discrete subpopulation of activated B cells.

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