Early pregnancy factor is immunosuppressive

IMMUNOSUPPRESSION during pregnancy has important consequences not only with respect to the recognition of the antigenically dissimilar embryo 1 but also in terms of susceptibility to viral or tumour assaults 2,3 . We have recently described an early pregnancy factor (EPF) detectable within 24 h of mating and have postulated that this protein is responsible for depression of lymphocyte activity during pregnancy 4–6 , Immunosuppressive properties have been attributed to other proteins detectable during pregnancy. These include pregnancy-associated macroglobulin 7,8 α -fetoprotein 9 and human chorionic gonadotropin (HCG) 10,11 . One characteristic these substances have in common, but in which they differ from EPF, is that they are not present, or not detectable, in the early stages of pregnancy. Recently the immunosuppressive activity of HCG has been disputed by several workers working with highly purified material 12,13 . We have now shown that EPF inhibits the expression of an immune response in vivo .