Q.Explain Rh-incompatibility in humans.

Rh incompatibility centres on the Rh antigen present on the red blood cell surface of about 80 per cent of people (Rh-positive), with the rest lacking it (Rh-negative). If an Rh-negative person is exposed to Rh-positive blood, their body forms antibodies against the Rh antigen, so Rh grouping must be matched before any transfusion, just like ABO grouping. The most serious form of this incompatibility occurs in pregnancy: an Rh-negative mother carrying an Rh-positive foetus is normally protected during the first pregnancy by the placenta, but a small leak of foetal Rh-positive blood at delivery sensitises the mother to produce anti-Rh antibodies; in a later pregnancy with another Rh-positive foetus, these antibodies can cross the placenta and destroy the foetal red blood cells, a condition called erythroblastosis foetalis that can cause severe anaemia, jaundice, or death, and which can be prevente …

Besides the ABO blood groups, human blood is also classified by the Rh antigen on the red blood cell surface, named after a similar antigen first observed in the rhesus monkey. About 80 per cent of people carry this antigen and are Rh-positive, while the remainder, lacking it, are Rh-negative.

Rh matching in transfusion. If the blood of an Rh-negative person is exposed to Rh-positive blood, that person's body treats the Rh antigen as foreign and begins to produce anti-Rh antibodies against it. A later exposure to Rh-positive blood would then trigger a harmful reaction, as these antibodies attack the transfused red cells — which is why Rh grouping must always be matched before a transfusion, exactly as ABO groups are.

Rh incompatibility in pregnancy. A special and serious case of this incompatibility arises during pregnancy, when an Rh-negative mother is carrying an Rh-positive foetus (having inherited the antigen from an Rh-positive father). The placenta normally keeps the two bloodstreams apart, so the first such pregnancy is usually safe. However, at the time of delivery, some of the foetal Rh-positive blood can leak into the mother's circulation, and she then begins forming anti-Rh antibodies. …

Method: Explaining an Antigen-Antibody Incompatibility Mechanism

Use this approach for any "explain X incompatibility" question involving an antigen and the antibody response to it.

Steps

Step 1: Identify the antigen and who has/lacks it

State which antigen is at the centre of the incompatibility and how the population splits -- here, the Rh antigen; about 80% are Rh-positive (carry it), the rest Rh-negative (lack it).

Step 2: State the general rule of exposure and response

Recall the underlying immunological rule that governs every such incompatibility: exposure of an antigen-lacking individual to that antigen triggers antibody formation against it; a later exposure then triggers an attack.

Step 3: Apply the rule to the specific scenario in the question …