Pregnancy-induced monoclonal antibody to a unique fetal antigen.

An IgM monoclonal antibody was produced by fusing maternal splenic lymphocytes obtained on the day of delivery of the first litter in a WF X DA mating with P3-653 myeloma cells, and at the time when the lymphocytes were isolated there was no circulating antibody in the pregnant animal. The monoclonal antibody had the same specificity as the antibodies in the postpartum pregnancy serum: it reacted with a unique class I MHC antigen, designated the pregnancy-associated (Pa) antigen, which is not one of the currently known, haplo-type-specific class I antigens. The use of recombinant strains mapped the locus encoding the Pa antigen to the region of RT1.A and to the left of RT1.B.