Question

why is it necessary to know the blood group before giving transfusion

Answer 1

Blood groups are actually the various or different type of bloods that have different antigen characteristics. The different blood groups are:-
1) A
2) B
3) AB
4) O
    
Other than blood group there is a Rh factor which gives the blood group positive sign if it is present in the blood and negative sign if it is absent.

Blood groups are important because they determine who can give blood to whom.

The people with the blood group 'O' are known as universal donors because they can give blood to every blood group but can accept blood from only 'O' type of blood group.

The people with blood group 'AB' are known as universal accept-or because they can accept blood from any blood group, but can give blood only to 'AB' type of blood group.

The people with blood group 'A' and 'B' can give blood to 'AB', 'A' and 'AB', 'B' respectively, and can accept from 'A', 'O' and 'B', 'O' respectively.

Also it has to be noted that the Rh factor also plays an important role in blood transfusion, i.e. people with same Rh factor can donate and accept blood to / from the people with same Rh factor.

Answer 2

then all the blood can be mixed and if a person's bloodgroup is A+ then if another person'  bloodgroup is B+ then other person for whom the blood is given then he may get diseases