Suppression of mixed lymphocyte reactions by pregnancy serum.

Maternal lymphocyte function, as assessed by stimulation with cells from a human lymphoid line, is normal in pregnancy. Maternal serum, however, contains immunosuppressive factor(s), demonstrable by 29 weeks of pregnancy, and having a greater effect at 36 weeks on mixed lymphocyte reactions. These immunosuppressive factor(s) block all mixed lymphocyte reactions to the same extent and are not specific towards those stimulated by paternal antigens. Anti-B cell (dr) alloantibodies, presumably directed against foetal antigens, are not uncommon in maternal serum even in early pregnancy. There is some suggestion that the presence of these antibodies in early pregnancy is associated with selective inhibition of maternal lymphocyte stimulation by paternal antigen. No correlation was found between the presence or absence of serum suppressive factors and the clinical course of the pregnancy.