Histologic changes occurring in the rabbit spleen during the primary response to intravenous injection of bovine gamma globulin alone and in combination with bacterial endotoxin have been described. The predominant morphologic changes associated with antibody formation in animals that received antigen alone were confined to the follicular areas of the spleen. Only minimal changes were seen in the red pulp. No new cellular types appeared following endotoxin stimulus. Rather, a profound augmentation of events observed following a single injection of bovine gamma globulin alone appears to account for the higher levels of antibody incited by this adjuvant. The cell type important in antibody formation under these conditions appeared to be a modified reticular cell. Plasma cells were not part of the cellular reaction. A hypothesis describing the primary antibody response is presented.

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