The primary immune response elicited by host antigens in a grafted population of immunologically competent cells has been compared in conditions where the same dose of parental cells were grafted simultaneously to F1 hybrid embryos of 13 or 17 days of age. The enlarged chimeric spleens harvested 4 days later were analyzed for donor cell proliferation by using the sex chromosomes as karyological markers, and for proliferation of immunological activity by means of transfer to secondary hosts of the same genotype.

Whereas the total number of dividing donor cells were on the average 16 times higher in 17-day than in 13-day hosts, the recovery of immunological reactivity showed a 6- to 7-fold difference in the opposite direction.

The experiments cast doubt on the proposition that cellular proliferation is necessary for development of a primary immune response. They suggest that there exists an alternative way in which a primary immune response may unfold from involving a few to involving a much larger number of cells.

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