Rh incompatibility, the agglutination (clumping together) of red blood cells as a result of mixing different antigens (agglutinogens) present on the surface of the cells. This agglutination is an immune reaction and depends on the formation of antibodies against the specific agglutinogen (Rh factor) present on the red blood cells and in blood from a transfusion or fetal tissues. The immune reaction does not occur immediately, but depends on the gradual formation of antibodies. For example, a D-negative (Rh negative) person who has been previously exposed to D-positive red cells through transfusion or pregnancy may produce anti-D, and therefore be incompatible with all D-positive (Rh positive) red cells. See also Rh factor.
