Question

Human blood cell typing why do blood types matter?

Answer 1

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.