The localization of antibody in the interior of plasma cells of lymph nodes of rabbits hyperimmunized with ferritin was studied by electron microscopy. The cells were incubated with the antigen (ferritin) which was allowed to penetrate into the cells by suitable methods. Antigen-antibody precipitates were localized in the cisternae of the endoplasmic reticulum and in the perinuclear space. No evident association was found between ferritin and ribosomes or ferritin and outer cell membrane. Cells from control animals immunized with an unrelated antigen, incubated with ferritin, exhibited no labeling. From direct counts of ferritin molecules in plasma cells, a lower limit was evaluated for the antibody concentration in the endoplasmic reticulum (12.2 mg/cm3) and for the total antibody content of a plasma cell (7.0 x 10–3 gm).

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