The present studies have shown that cells capable of specific localization in response to challenge with CRBC or SRBC synthesize DNA very rapidly during the period from 2–5 days (peak 3 days) post primary immunization. This has been done by incubating the antigenically stimulated lymphoid cells with [3H] or [14C]thymidine in vitro for 45 min before adoptive transfer to syngeneic recipients. Specifically localizing cells (SLC) labeled in this way may ultimately account for up to 50% of the 3H or 14C present in a set of specifically challenged lymph nodes 3 days later. The data presented are consistent with the hypothesis that SLC numerically constitute only a very small fraction of the total number of recirculating lymphocytes trapped in antigenically stimulated lymph nodes, and that the demonstration of specific localization therefore depends upon selectively labeling these SLC relative to other recirculating cells. Attempts to selectively label the RNA of SLC with the precursor uridine have to date met with only very limited success.

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