Give information about $Rh$ blood group.

(A) The $Rh$ antigen was first discovered in $Rhesus$ monkeys,hence it is named the $Rh$ factor.
- This antigen is also observed on the surface of $RBCs$ in most humans. Individuals possessing this antigen are called $Rh$ positive $(Rh^+)$,while those lacking it are called $Rh$ negative $(Rh^-)$.
- Approximately $85\%$ of the human population is $Rh$ positive,and $15\%$ is $Rh$ negative.
- An $Rh^-$ person,if exposed to $Rh^+$ blood,will form specific antibodies against the $Rh$ antigens. Therefore,$Rh$ compatibility must be checked before blood transfusion.
- $A$ special case of $Rh$ incompatibility occurs between the $Rh^-$ blood of a pregnant mother and the $Rh^+$ blood of the foetus.
- During the first pregnancy,$Rh$ antigens of the foetus are not exposed to the mother's $Rh^-$ blood because the two blood supplies are separated by the placenta.
- However,during delivery,the mother may be exposed to the foetal $Rh^+$ blood,causing her to produce antibodies against the $Rh$ antigen.
- In subsequent pregnancies,these $Rh$ antibodies from the $Rh^-$ mother can cross the placenta and enter the $Rh^+$ foetal blood,destroying the foetal $RBCs$. This can be fatal to the foetus.
- This condition causes severe anaemia and jaundice in the baby and is known as erythroblastosis foetalis.
- This can be avoided by administering anti-$Rh$ antibodies to the mother immediately after the delivery of the first child.