The physical presence of the antigen used to stimulate a secondary antibody response was demonstrated in the cells of popliteal lymph nodes. Rabbits previously injected with apoferritin (containing no iron), which was prepared from recrystallized horse ferritin, were given an injection of ferritin 5 weeks later. The antigen was traced by means of the Prussian blue reaction, by specific fluorescent antibody, and by electron microscopy. Antiferritin antibody was localized by immunofluorescence, though it was not possible to test cells simultaneously for antigen and antibody. Horse ferritin induces a rather weak primary antibody response, but a brisk secondary response characterized by the appearance in the medullary cords of numerous plasma cells containing antiferritin. Many intact ferritin molecules were found in the nucleus and cytoplasm of numerous reticular and other phagocytic cells in the sinuses. In decreasing amount, ferritin molecules were also clearly demonstrated in hemacytoblasts (plasmoblasts), and immature and mature plasmocytes.

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