1. The immunological mechanisms involved in sustaining normal semi-allogeneic pregnancies and in the enhancement of organ allografts were investigated in inbred rats.

2. The antigenic targets for alloantibodies formed after leucocyte transfusions and multiple allogeneic pregnancies were defined by the EA rosette inhibition (EAI) assay in several congenic and recombinant inbred rat strains.

3. Alloantibodies produced by leucocyte immunization (conventionally induced antisera) were directed only to RT1-encoded (major histocompatibility complex, MHC) antigens. Both RT1A (class I MHC) and either RT1B, D (class II MHC) or RT1C (Qa-like) antigens were targets for these alloantibodies; responses to the latter three antigens could not be separated with available congenic recombinant inbred rat strains.

4. Alloantibodies produced as a consequence of multiple semi-allogeneic pregnancies (pregnancy-induced antisera) were directed only to RT1A antigens.

5. Allogeneic pregnancies in which the paternal strain differed from the maternal strain only at the RT1A gene locus produced suppression of a subsequent maternal immune response.

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