A study of the kinetics of antibody-producing cells has been carried out by the use of rosette formation for detection of individual antibody-producing cells, and labeling with tritiated thymidine, in cells obtained from mouse spleens at intervals after injection of SRBC. Following a primary injection of the antigen, the number of RFC per million cells was found to increase to a peak at 5 days, then, after a decrease, to a second peak at about the 10th day. The curve of tritium labeling of RFC was also biphasic, with peaks on the 3rd and 7th day. The second increase in rosette-forming cells could be shown to involve, especially between the 7th and 9th day, a second increase in lymphoid cell RFC and, among these, 7S antibody-producing cells. When the population examined was restricted to large lymphocytes, two peaks of RFC per million cells and two peaks of labeling were again found. In this case, however, the peaks of RFC and of labeling were reached on the same day in each instance, rather than with the 2 day difference found in the entire spleen cell suspension or the entire lymphoid cell population.

Electron microscopic examination of labeled rosette-forming cells showed these to be largely lymphocytes, but to include rather well differentiated plasmablasts as well. No macrophages were found among labeled RFC in the primary response. A substantial number of labeled lymphocytes were found in close contiguity with rosette-forming macrophages. The percentage of labeling in such lymphocytes was as high, on the respective days, as the percentage of labeled cells among the RFC of the entire suspension.

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