Cell suspensions from draining lymph nodes of immune and nonimmune rats were reacted in vitro with 125I-labeled antigens. In light microscopic radioautographs of smears, 17% of the immunized cells were tagged by specific antigen; 2.0% of control cells were positive. In electron microscopic radioautographs, 90% of the labeled elements from immune donors were lymphocytes, blast and plasma cells; 10% were monocytes-macrophages or other elements, including naked nuclei. 15% of the labeled cells from control materials were lymphocytes and plasma cells, while 85% were monocytes-macrophages and naked nuclei.

Within cell suspensions derived from immunized animals there were almost twice as many lymphocytes marked by isotope as plasma cells, and the lymphocytes ranged in morphology from mature monoribosomal elements to immature polyribosomal cells. Antibody-forming cells fixed labeled antigen at their surfaces. The monocyte-macrophage class was distinguished by a high mean grain count and by distribution of grains within cytoplasmic vacuoles and lysosomes.

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