In contrast to plasma cells which are known to synthesize their own immunoglobulin, the role of the small lymphocyte is less clear. Although it has been postulated that it has the ability to develop into a plasma cell and thus become immunologically active, Lobb1 has shown that differentiation into a plasma cell may not be a prerequisite for the synthesis of immunocompetent globulin.
Human peripheral lymphocytes were cultured in a short-term tissue culture in the presence of phytohemagglutinin. In order to detect cellular immunoglobulin, the culture was treated with fluorescent antisera against the immunoglobulin heavy chain markers Gm(a) and Gm(b) and also against whole human immunoglobulin-G. All cells absorbed at least one marker; however, not all absorbed the same one. There were two cell populations, one positive with the Gm(a) serum and the other positive with the Gm(b) serum. This provides strong evidence that immunoglobulin was synthesized in the