Human blood cell typing why do blood types matter?

Blood typing is important in the blood transfusion. 
Losing too much blood will result in death. The easiest way to prevent death by loss of blood would be doing a blood transfusion. Blood transfusion is injecting blood from other people to a patient that losing too much blood.
Human has antigen in their blood that can be recognized as a foreign object and get attacked. If you give someone a blood that has similar antigen which he has, the blood will not be recognized as foreign material. But if you do, the antigen in the blood will induce hypersensitivity reaction.