Time of appearance and distribution of cells capable of secondary immune response following primary immunization.

Immunological memory was studied by measurement of tritiated thymidine incorporation in tissue culture. After primary immunization with keyhole limpet haemocyanin (KLH) secondary responsiveness could be detected as early as the 2nd day after immunization with Freund's adjuvant into the footpads and on the 4th day after injection of KLH intravenously. In each case immunological memory developed first in the area of the injection, that is, the popliteal lymph nodes after footpad immunization and the spleen after intravenous injection. The secondary response could also be detected in the lymphoid cells of the blood. Cell suspensions enriched in small lymphocytes showed a similar reactivity. Cells from the thymus, however, did not develop immunological memory. Rabbits immunized with BSA showed a relatively weaker response which was clearly detectable only when Freund's adjuvant was used for immunization. The results suggest that a response essentially of a secondary type may play an important role in what is usually considered the primary immune response.