If all living systems, not plants, have blood; then why do blood types matter?

because when humans get blood transfusions they need the same type of blood in their body or they will reject it and die.

Explanation:

each blood type has certain antigens and antibodies that will only accept a certain blood type. type O can give to all blood types (A, B, AB, O) but can only receive from type O. Type AB can receive from A, B and O because it has both A and B antigens. if you were to give type A to a type B person the type B antibodies would attack the foreign type A with its anti-A antibodies.

Prevent risks

Explanation:

The importance of knowing your blood type is to prevent the risk of you receiving an incompatible blood type at a time of need, such as during a blood transfusion or during surgery. If two different blood types are mixed, it can lead to a clumping of blood cells that can be potentially fatal.