The reactivity of lymphoid cells from germfree and conventional CDF rats were compared in mixed leukocyte culture (MLC) with both allogeneic rat leukocytes and with human leukocytes as stimulating cells, and in a graft-versus-host (GVH) assay. There was no difference between germfree and conventional rats in any of these systems. Furthermore, the increase in GVH activity after specific immunization against a strong histocompatibility antigen was small in both cases. The findings are incompatible with the hypothesis that the high proportion of antigen-sensitive cells against strong histocompatibility antigens is caused by cross-reactions between microbial antigens and histocompatibility antigens.

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