Why is it necessary to match the donor's and the recipients blood before a transfusion is given

It is necessary to match the donor's and the recipient's blood before a transfusion because the immune system will react badly if blood cells are not compatible. The immune system is responsible for patrolling or for guarding the circulation in the body. Without its proper function, the body will be exposed into different types of infections. 

When blood goes through the process of transfusion, the cells are treated the same way as infections or foreign invaders. When they are not compatible with the recipient the immune system will be massively activated which can cause immediate clotting systems in the blood. The results of this clotting can lead to shock, kidney failure, collapse of the circulatory system, and worst death.