The Rh system and Pregnancy
If a Rh-ve woman conceives Rh +ve baby (husband being Rh +ve) then during childbirth some foetal RBCs with D antigens can pass to the mother. In the mother they produce anti-D antibodies. In the next pregnancy if there is Rh +ve foetus (which is usual), these antibodies from the mother will (these antibodies are smaller and can pass through the placental barrier) lead to destruction of the foetal RBCs leading to haemolytic disease of the newborn (HDN). If the mother is already having anti-D antibodies due to previous transfusion (see above) then the first baby will be affected, HDN is otherwise rare in the first pregnancy (though it can occur in first pregnancy due to foetal-maternal haemorrhage during pregnancy in some cases). The chance of HDN increases with each subsequent pregnancies.
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In mild degree of destruction of foetal red cells, it leads to anaemia and rapid RBC production, therefore, blast cells (erythroblasts) come into circulation and this is called erythroblastosis foetalis. It is also associated with jaundice.
In severe degree of foetal RBC destruction more intense jaundice occurs (icterus gravis neonatorum). When the bilirubin level is 19mg/di, bilirubin crosses the blood-brain barrier (which is not developed properly in foetus) and is deposited in the basal ganglia leading to kernicterus characterised by increased muscle tone, etc.
In the severest form of HDN, hydrops foetalis develops, when the foetus is usually dead. It is characterised by severe oedema and swelling of the whole body of the foetus.
HDN can also occur in ABO incompatibilities when the mother having O group and the foetus A or B, but it is usually of mild type.
Treatment:
The haemolytic disease of the newborn can be prevented if the mother (Rh -ve) is injected with anti-D antibodies immediately (within 72 hrs) after the childbirth. These antibodies then destroy the Rh +ve RBCs of the foetus which may have entered into the maternal circulation. This prevents development of anti-D antibodies in the mother. Anti-D antibodies are also injected during pregnancy but opinion differs about how much to be injected and when to inject.
If the baby is born with the disease then the jaundice is treated by photo therapy, the bilirubin is converted in to lumirubin and is excreted through the liver easily.
Treatment by exchange transfusion may be needed in more sever cases. Here the blood of the foetus is fully replaced several times in the first few weeks by fresh blood, small amount at a time. The blood used is Rh -ve and properly matched for ABO system though the neonates have few or no agglutinins.
Coomb's test-
In HDN, the Rh antibodies from mother's blood pass through the placenta and combine with the Rh antigens on the foetal RBCs. Such RBCs are called sensitised RBCs. Coomb's test is used to detect these sensitised RBCs and also to detect Rh antibodies in mother's blood. In this test a serum containing antibodies against human globulin i.e., anti-antibodies is used. This serum, called coomb's serum is produced in rabbit by injecting globulin (ordinary serum) repeatedly.
When this coombs serum is mixed with foetal blood the sensitised RBCs agglutinate because the antiglobulin antibodies combine with the Rh antibodies on the sensitised RBCs. This is called direct coomb's test.
In indirect coomb's test, mother's serum is mixed with group ORh+ve RBCs and then treated with coomb's serum. If agglutination occurs then it proves that mother's serum contains Rh antibodies.