Well, at least we didn't go down without explaining ourselves first. The difference in blood group here does not mean the ABO blood group system, ie if the mother's blood group is A and the father's blood group is B, it will not make any difference.
But here we will look at the Rh system, ie blood group is positive or negative. Now, like the rest of the blood groups, the Rh blood group also depends on the genetics, that is, if someone's genes contain the Rh factor gene, then their blood group will be + if not, then it will be -.
Now suppose that a woman who is of - blood group and a man who is of + blood group, both got married, and after marriage when their child is in the mother's womb then the child will be of + blood group , Because the child's father can get the + genes.
That is, now the situation is that the mother is of blood group but in her womb the child is of + blood group. At the time of delivery, some of the baby's blood from the placenta enters the mother's blood, what happened? Now there is + blood group in mother's blood group as you will know that when a person of one blood group includes different blood group, his immune system becomes active. This will make antibodies for this opposing group in the mother's body, but the baby is delivered, so there is no problem for the first baby. But suppose this woman is conceived again by the same husband and again the woman with blood group has a child with + blood group, so what will happen now? Now the immune response and antibodies that were made at the time of the first delivery will attack the baby, and the baby's growth will be abnormal, then more blood will be added at the time of delivery, thus with each new pregnancy. The baby will have an abnormality (Erythroblastosis fetalis) and the baby may die.
But don't worry, there is a medical treatment for this, the baby's blood group is checked as soon as the baby is born, and if the mother and baby's blood group is Rh factor different, it is because of the blood that flows during delivery. Medicine is given to eliminate the antibodies that are formed.
But it doesn't have to be with every - and + pair. If the father has a heterozygous condition, then half of his sperm + blood group genes will be kept and half - of the blood group, then if the child - blood group genes from sperm If it comes into existence, there will be no problem ...
I hope I have succeeded in explaining my point ...
Another question is why the ABO blood group system does not have this problem? This is because the antibodies in this condition are usually IgM, which, due to its size, cannot pass through the placenta and reach the baby.