The present studies are based on previous observations that the intravenous injection of diphtheria toxoid and complete Freund's adjuvant into rabbits resulted in an increased proliferation of cells associated with antibody synthesis; an accelerated, enhanced, and prolonged synthesis of antibody; and a lengthened interval between the appearance of γM- and γG-hemagglutinating antibodies in the circulation. The molecular species of antibodies that were synthesized by fragments of the spleens were determined by the incorporation of labeled amino acid into antibody and by binding of radioactive antigen by antibody. These studies were paralleled by determination of the presence and type of antibody within the cell by immunofluorescence. Evidence was obtained that non-phagocytic mononuclear cells in the walls of the sinusoids of the red pulp of the spleen are a major source of 19S γM-antibody and plasma cells in the non-follicular white pulp are a major source of γG-antibody. The data did not exclude the synthesis of γG-antibodies by the mononuclear cells, the synthesis of γM-antibodies by the plasma cells, or the synthesis of both antibodies by an occasional cell of either morphology. It was hypothesized that the 19S and 7S antibody responses evolved independently with the development of at least two different cell types, a mononuclear cell with capacity for 19S immunoglobulin synthesis and a plasma cell with capacity for 7S immunoglobulin synthesis.

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