Blood lymphocytes from rabbits immunized against BSA were stimulated by this antigen in vitro and their response determined by the increase in uptake of 3HT. The rabbits were immunized with different doses of antigen in complete adjuvant or in saline solution and the lymphocytes, collected at intervals of 3 to 270 days were stimulated in vitro by concentrations of antigen, ranging from 10ndash;ndash;2mu;g to 2 times; 104mu;g/culture. Lymphocytes from rabbits immunized with 20 mg BSA in adjuvant showed a marked increase in sensitivity toward the antigen (up to 105-fold) when collected at intervals up to a month after immunization, while lymphocytes from rabbits immunized with 0.5 mg BSA in adjuvant reached a peak by day 16 and decreased thereafter. This increase was expressed by a decrease in the minimal dose of antigen needed to stimulate a significant response. The increase in sensitivity was faster and the levels of response were higher in rabbits immunized with 0.5 mg than with 20 mg. Lymphocytes collected at different intervals late after immunization with the above doses of antigen showed periodical changes in the levels of response with alternating high and low responses. Cells from animals immunized with 10 mu;g BSA in adjuvant, on the other hand, showed low levels of response and a diminished increase in sensitivity, compared to cells from the other 2 groups. Cells from rabbits injected with BSA in saline did not react to antigen in culture. This form of antigen was found also to induce a partial unresponsiveness as shown by decreased responses after subsequent challenge with antigen in adjuvant.
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