The role of proliferation in the development of 19S antibody-forming cells in the primary response has been investigated in an in vitro system.

Spleen cell suspensions from normal, unimmunized mice were cultured in vitro in the presence of mammalian erythrocytes and the number of 19S hemolytic plaque-forming cells that arose 4 days later was measured. The hot pulse technique for the selective irradiation of those cells which synthesize DNA during a defined period of time has been described.

The effect of such hot pulses administered at various times after the addition of antigen on the subsequent appearance of antibody-forming cells was determined.

The results established that: (a) the onset of DNA synthesis does not start for approximately 24–32 hr after the addition of antigen, (b) essentially all the antibody-forming cells arise by cell division, and (c) different cell populations are involved in the response to two non-cross-reacting antigens.

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