Pairs of rats were immunized with keyhole limpet hemocyanin (KLH) and simultaneously labeled with thymidine-methyl-3H or 5-iodo-2'-deoxyuridine-125I. From 10–50 days later, their lymphoid organs were examined 3 days after anamnestic stimulation with KLH or after primary injection of BGG. Light and electron microscopic study of the labeled cells revealed that immunologic memory resided in the mature resting monoribosomal lymphocyte which, upon stimulation, transformed to an immature polyribosomal lymphocyte and mitotically active blast cell. These latter elements differentiated into plasma cells directly or after mitosis.

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