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ABO grouping and Rh incompatibility
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The major importance of the Rh system for human health is to avoid the danger of RhD incompatibility between mother and fetus.
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Problems with the Rh factor occur when the mother's Rh factor is negative and the baby's is positive. Sometimes, an incompatibility may occur when the mother is blood type O and the baby is either A or B.
Rh blood type incompatibility is still the leading cause of potentially fatal blood related problems of the newborn. In the United States, 1 out of 1000 babies are born with this condition.
The Rh System
Rh antigens are trans membrane proteins with loops exposed at the surface of red blood cells, used for the transport of carbon dioxide and/or ammonia across the plasma membrane. They are named for the rhesus monkey in which they were first discovered.
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Father's Rh factor genes
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Mother's Rh factor genes
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Baby
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+ +
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+ +
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+ + (Rh positive)
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+ +
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- -
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+ - (Rh positive)
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+ -
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+ -
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+ + (Rh positive)
+ - (Rh positive)
- - (Rh negative)
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_ _
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+ -
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+ - (Rh positive)
- - (Rh negative)
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Fetal- maternal transfusion
Some mothers may be sensitized by fetal-maternal transfusion of ABO incompatible red blood and produce immune IgG antibodies against the antigen they do not have and their baby does. For example, when a mother of genotype OO (blood group O) carries a fetus of genotype AO (blood group A) she may produce IgG anti-A antibodies. The father will either have blood group A, with genotype AA or AO, or more rarely, have blood group AB, with genotype AB.
Mother-fetus incompatibility
During birth, there is often a leakage of the baby's red blood cells into the mother's circulation. If the baby is Rh positive and the mother Rh-negative, these red cells will cause her to develop antibodies against the RhD antigen. The antibodies, usually of the IgG class, do not cause any problems for that child, but can cross the placenta and attack the red cells of a subsequent Rh+ fetus. This destroys the red cells producing anemia and jaundice. The disease, called erythroblastosis fetalis or hemolytic disease of the newborn, may be so severe as to kill the fetus or even the newborn infant. It is an example of an antibody-mediated cytotoxicity disorder.
The first time an Rh- woman becomes pregnant, there usually are not incompatibility difficulties for her Rh+ fetus. . When the next pregnancy occurs, a transfer of antibodies from the mother's system once again takes place across the placental boundary into the fetus. The anti-Rh+ antibodies that she now produces react with the fetal blood, causing many of its red cells to burst or agglutinate. As a result, the newborn baby may have a severe life-threatening anemia. The risk increases with each birth.
Prevention
- Erythroblastosis fetalis can be prevented for women at high risk (i.e., Rh- women with Rh+ mates or mates whose blood type is unknown) by administering a serum (Rho-GAM) containing anti-Rh+ antibodies into the mother around the 28th week of pregnancy and again within 72 hours after the delivery of an Rh+ baby.
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