Rh alloimmunization is an immune-mediated process triggered when Rh-negative maternal blood is exposed to Rh-positive fetal erythrocytes during fetomaternal hemorrhage (FMH). Initial exposure produces a primary immune response with IgM anti-D antibodies that are too large to cross the placenta. Subsequent exposure in a future pregnancy elicits a rapid anamnestic IgG response. These IgG anti-D antibodies cross the placenta and bind to Rh (D) antigens on fetal RBCs, marking them for destruction by the fetal reticuloendothelial system. Progressive hemolysis causes fetal anemia, extramedullary hematopoiesis (hepatosplenomegaly), hyperbilirubinemia, hypoalbuminemia, and in severe cases, hydrops fetalis with high-output cardiac failure. The nurse must monitor antibody titers, coordinate fetal surveillance, administer RhoGAM prophylaxis, and manage the newborn with hemolytic disease.