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Rh factor 'är-'āch- n a genetically determined protein on the red blood cells of some people that is one of the substances used to classify human blood as to compatibility for transfusion and that when present in a fetus but not in the mother causes a serious immunogenic reaction in which the mother produces antibodies that cross the placenta and attack the red blood cells of the fetus called also rhesus factor
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Rhesus factor any of numerous antigens (agglutinogens) that may be present on the membrane of erythrocytes and that determine the Rh blood group system; the most common ones are called (in one system) Rh 1, Rh 3, Rh 4, Rh 5, and Rh 21. See Rh blood group, under blood group. Called also Rh antigen.Medical dictionary. 2011.