Mice were immunized intravenously with 4 × 107 thymocytes from Thy-1 disparate, either H-2-compatible or H- 2-incompatible donors. The magnitude of the anti-Thy-1.1 response was measured by determining the number of PFC in spleens of animals 6 days after immunization. Regardless of the origin of immunizing and target thymocytes, the assay employed detected exclusively PFC-producing antibodies to the Thy-1.1 antigen. In almost all instances, H-2-compatible thymocytes elicited a significantly higher response than did H-2-incompatible thymocytes, although the latter occasionally evoked a high response. The H-2 incompatibility between donor and recipient appeared to be responsible for the differences in responsiveness of the standard inbred mice and their H-2 mutants immunized with thymocytes compatible with standard inbred strains. The phenomenon observed appears to have several features in common with antigenic competition. We propose that the requirement for H-2 compatibility in the anti-Thy-1.1 response may be the expression of a general requirement of T cells to recognize an antigen in the context of the H-2 molecule.
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