['Class II major histocompatibility antigens occupy a central role in the development of humoral or cellular immunologic responses. Surprisingly, in the maternal-fetal interphase, where two genetically different organisms come in direct contact, these antigens are absent. Based on previous studies in mice we have demonstrated that the absence of class II antigens represents another mechanism of fetal protection from the maternal immune response and, furthermore, that the induction of these antigens in the placenta makes the tissue immunogenic and susceptible to maternal immune attack, leading thus to fetal abortion. In order to test this hypothesis in humans, we analyzed the presence of class II antigens in aborted placentae.', 'Class II expression on aborted placentae was examined by immunoperoxidase staining on frozen sections. We also studied hematologic changes that accompany such events, measuring white and red blood cell counts, hematocrit, hemoglobin, as well as IgG serum levels by standard techniques.', 'The detection of class II antigens on the aborted tissue indicates that these antigens are indeed major components of a pathway, which leads to a plethora of abnormal phenomena for the maternal organism such as low hematocrit levels, elevated IgG production, and increase of white blood cell numbers.'] Does placental tissue from human miscarriages express class II HLA-DR antigens?