The phenotype of three large cell (histiocytic) lymphomas of man has been defined with convential lymphocyte surface marker techniques, a panel of monoclonal antisera of hybridoma derivation (Ortho Pharmaceutical Corp.), and the fluorescence microscope. Two tumors exhibited surface IgG of lambda light-chain type, whereas the third was negative for surface Ig but contained the same Ig in the cytoplasm. With the monoclonal antisera, all three were found to bear two surface antigens (OKT9 and OKT10) previously described on early thymocytes, and were devoid of the Ia antigen. The available knowledge suggests that these neoplasms represent the expansion of a clone of B lymphocytes along the pathway that leads to plasma cell differentiation rather than a clone with both B and T surface membrane markers.

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