Rat lymphocytes in mixed cultures can reutilize tritiated thymidine from labeled granulocytes. Shortly after thymidine injections in vivo, major effects on the frequency of labeled lymphocyte mitoses in peripheral blood cultures are introduced by 10-20% polymorph contamination, even though transfer of label via supernates is not demonstrable. Cold thymidine in the cultures prevents reutilization, and has permitted reevaluation of several previous conclusions concerning the life history of lymphocytes reactive to major histocompatibility alloantigens (HARC). Rather than being predominantly recently divided cells, HARC do not appear to have an age distribution, in blood or lymph, significantly different from the general recirculating lymphocyte population. However, the ability of immunization across strong allogeneic differences to increase markedly the proportion of young HARC among the specifically responsive population has been confirmed.

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