Decreased antigenicity of mouse blastocysts after activation for implantation from experimental delay

The antigenicity of mouse blastocysts in experimentally delayed implantation and after activation from delay by estradiol administration were determined by two different methods. CBA blastocysts were incubated in C57BL/6J anti-CBA serum, and the amount of antibodies bound to the blastocysts was traced by -125-I-conjugated sheep antimouse gamma-globulin (isotope antiglobulin technique) and sensitized sheep red cells (mixed haemadsorption technique). With both techniques it was possible to demonstrate that mouse blastocysts in experimental delay of implantation possess alloantigens, and that this antigen expression has decreased markedly 14 hr after activation for implantation. It is suggested that this phenomenon may be of significance for noncognition of the allogeneic conceptus during pregnancy.