A technique is described which makes it possible to detect individual antibody-forming cells using a localized hemolysis reaction in a thickened culture medium containing sheep erythrocytes and guinea pig complement. This technique has the advantage over single cell isolation in that it is technically feasible to survey large populations in order to detect a very small active fraction. The cells can be observed continuously during the time of antibody release, and it appears that an estimate of the relative antibody-forming activity can be made from the size of the areas of lysis. Experiments with metabolic inhibitors indicate that active synthesis is occurring rather than release of preformed antibody. Some experiments on the detection of antibody other than anti-red cell antibodies are reported.

This technique has been applied to a study of the induction period of the primary response of rabbits to sheep red blood cells. The results of this experiment are consistent with an induction period of 2 to 3 days during which there is no increase in the number of active cells in spleen and lymph node reflecting the lag in appearance of detectable serum antibody followed by an abrupt rise of 50- to 100-fold between the 3rd and 5th day. However, the present data are not sufficient to exclude various other mechanisms.

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