/ahr"aych" fak'teuhr/ , Physiol.
any of a type of specific antigen present on the surface of red blood cells, persons having inherited such antigens being designated Rh+ (Rh positive) and persons lacking them, a much smaller group, being designated Rh- (Rh negative): blood of Rh- persons is incompatible with Rh+ blood because of antibody reaction, and an Rh- woman who bears an Rh+ baby will have formed antibodies to the fetal blood that, unless removed from her bloodstream by apheresis in a subsequent pregnancy, will be carried across the placenta and destroy red blood cells of the next Rh+ fetus, resulting in erythroblastosis in the newborn.
[ 1940-45; so called because first found in the blood of rhesus monkeys ]