Cellular events during the expression of immunologic memory.

A series of experiments were designed to identify cytologically the cell endowed with immunologic memory and to depict morphologically the cellular events that occurred during the anamnestic response. With the use of radioactive labels and autoradiography at the light and electron microscopic levels, the concatenated phenomena from activation of memory cells to production of antibody could be followed. The memory cell was found to be a lymphocyte which, when activated by specific antigen, enlarged and transformed into either a blast cell or an immature plasma cell. Accompanying this transformation was a boost in proliferative activity of blast and immature plasma cells. This augmented mitotic activity was significantly greater than the level determined in control situations. Following the proliferation among labelled blast and immature cells, maximal labelling occurred in temporal sequence among plasma cells and large lymphocytes and finally among small lymphocytes. Antibody-forming cells were detected chiefly among lymphocytes and plasma cells and increased over the 4 days during which lymphocytes and plasma cells were differentiating from precursor immature elements. The primary response to antigen simulated in many respects the secondary response, but at a significantly lower level.