In: Anatomy and Physiology
why is it possible for a man with Blood type A+and a woman with Blood Type B+ to have a baby with type O- blood?
If the blood group of the baby is O- , it means that the red blood cells (erythrocytes) of the baby lack both antigen A and antigen B of ABO blood grouping system as well as lacks Rh antigen of Rh blood grouping system. As the baby's erythrocytes lack antigen A, antigen B and Rh antigen, there will not be any incompatibility reaction between the mother's blood and baby's erythrocytes.
Hemolytic disease of newborn (HDN) or erythroblastosis fetalis occurs when there is either ABO or Rh incompatibility between the mother and the fetus. In such cases of incompatibility, the maternal blood produces IgG antibodies against fetal erythrocytes which causes hemolysis of fetal erythrocytes by maternal antibodies resulting in severe fetal anemia which can prove to be fatal for the fetus. Maternal blood starts producing IgG antibodies against incompatible fetal erythrocytes whenever there is feto-maternal hemorrhage and consequent exposure of fetal erythrocytes to maternal blood such as during delivery of the baby or abortion or rupture of placental membranes due to any cause. Chances of hemolytic disease of newborn is high when the mother is Rh- and the fetus is Rh+.
If the baby's blood group is O- there is neither antigens A and B nor Rh antigen to sensitize maternal blood to produce IgG antibodies against foetal erythrocytes. Thus, there is no chance of hemolytic disease of newborn in such case. Hence, it is safe for a mother with blood group B+ to have a baby with blood group O-.