The proliferative interaction of cultured rat lymphocytes of immunogenetically disparate origin—the mixed lymphocyte interaction—was employed as an experimental model to examine the initial stages of the immune response mechanism. Using mixed cultures of cells derived from parental strain and F1 hybrid rats, in which only the parental lymphocytes respond, the following observations were made on the magnitude and kinetics of the reaction.

After initiation of the cultures, there was a latent period of approximately 40 hours during which time no mitotic activity was detected.

This inactive phase was followed by a period of proliferation in which previously nondividing cells entered the mitotic cycle for the first time. Activity in the cultures, as detected by incorporation of radioactive thymidine and measured by radioautography or scintillation spectrometry, increased exponentially with a doubling time (T2) of 9–10 hr.

In this exponential proliferative phase, lasting approximately 100 hr, the dividing cells underwent a series of rapid sequential divisions with a generation time (Tc) of 8 hr, and few, if any, dropped out of the mitotic cycle.

In addition to the cells which first entered mitosis at the beginning of the proliferative phase and then proceeded through multiple divisions, significant numbers of new, previously nondividing cells continued to enter the mitotic cycle during the entire exponential growth phase. The total number of these newly responsive, first division cells throughout the total culture period amounted to 1–3% of the original parental cell inoculum. This is a surprisingly large proportion of peripheral blood lymphocytes with demonstrable reactivity to a particular antigen system, if it is assumed that these first division cells in vitro are functionally related to the hypothetical antigen-sensitive cells which proliferate and differentiate into immunological effector cells. At present there is no entirely satisfactory explanation for this large number of reactive cells in the mixed lymphocyte interaction.

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