The ABO system is based on the presence of A and B antigens in erythrocytes. ABO reagents give a weaker reaction in newborns than in adults. The weak reaction is due to the incomplete formation of A and B antigens at birth. Adult sera contain antibodies to A and B antigens. Since these antibodies appear 4-6 months after birth, reverse grouping is not performed on newborn blood samples. Rh D determination is based on the presence of D antigen in erythrocytes. If the mother is Rh D negative, it is important to determine the Rh D status of the newborn. The direct Coombs test is used to detect antibodies bound to the erythrocyte surface.