Write a short note on $ABO$ blood grouping.

(A) There are more than $30$ known $RBC$ antigens found in human beings,which classify the blood corpuscles of a particular person into different blood groups.
Among these,the two major types of blood groups are $ABO$ and $Rh$.
$ABO$ grouping is based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the $RBCs$,namely $A$ and $B$.
Similarly,the plasma of different individuals contains two natural antibodies. These are opposite to the antigens and are called anti-$A$ and anti-$B$.
Based on the presence of these antigens and antibodies,four blood groups,namely $A, B, AB,$ and $O$,are found in human beings.
The donor's compatibility is based on the antigens on the $RBCs$ and antibodies in the plasma,as shown in the table below:

Blood Group	Antigens on $RBCs$	Antibodies in Plasma	Donor's Group
$A$	$A$	anti-$B$	$A, O$
$B$	$B$	anti-$A$	$B, O$
$AB$	$A, B$	nil	$AB, A, B, O$
$O$	nil	anti-$A, B$	$O$

During blood transfusion,any blood cannot be used; the blood of a donor must be carefully matched with the blood of a recipient.
Group $O$ blood can be donated to persons with any other blood group,and hence,$O$ group individuals are called universal donors.
Persons with $AB$ group can accept blood from persons with $AB$ as well as the other groups of blood. Therefore,such persons are called universal recipients.