Explain Rh-incompatibility in humans.

Rh antigen is observed on the surface of RBCs of majority (nearly $80\%$ ) of humans. Such individuals are called $\mathrm{Rh}$ positive $\left({\mathrm{Rh}}^{+}\right)$and those individuals where this antigen is absent are called Rh negative $\left({\mathrm{Rh}}^{-}\right)$.

Both ${\mathrm{Rh}}^{+}$and ${\mathrm{Rh}}^{-}$individuals are phenotypically normal. The problem in them arises during blood transfusion and pregnancy.
(i) Incompatibility During Blood Transfusion The first blood transfusion of ${\mathrm{Rh}}^{+}$blood to the person with ${\mathrm{Rh}}^{-}$blood causes no harm because the ${\mathrm{Rh}}^{-}$person develops anti Rh factors or antibodies in his/her blood.
In second blood transfusion of ${\mathrm{Rh}}^{+}$blood to the ${\mathrm{Rh}}^{-}$person, the already formed anti $\mathrm{Rh}$ factors attack and destroy the red blood corpuscles of the donor.
(ii) Incompatibility During Pregnancy If father's blood is ${\mathrm{Rh}}^{+}$, mother blood is ${\mathrm{Rh}}^{-}$and the foetus blood is ${\mathrm{Rh}}^{+}$. it will lead to a serious problem. Rh antigens of the foetus do not get exposed to the Rh ${}^{-}$ve blood of the mother in the first pregnancy as the two bloods are well separated by the placenta.
But in the subsequent ${\mathrm{Rh}}^{+}$foetus, the anti Rh factors (antibodies) of the mother destroy the foetal red blood corpuscles due to mixing of blood.
This result in the Haemolytic Disease of the New Born (HDN), called as erythroblastosis foetalis. In some cases new born may survive but will be anaemic and may also suffer with jaundice.

This condition can be avoided by administering anit-Rh antibodies to the mother immediately after the delivery of the first child.