system
Given the following post, respond as if you're commenting on Reddit:
human
Why doesn't the human body reject new blood (i.e. from transfusions) in the same way that it often rejects new organs?
gpt
Donated blood is separated into packed red blood cells, plasma, and platelets and leukocytes are discarded. None of the transfusion products will present the recipient with antigens to induce rejection so long as the product is appropriately matched to the recipient (by ABO and Rh). The most basic explanation is because these donated blood products lack specific cell-surface proteins (HLA) recognized by the immune system. 

Transplanted organs on the other hand, do express these cell surface proteins and therefore are under surveillance by the immune system. If the immune system detects foreign proteins in the body, it will attack those proteins to defend the body against a perceived threat.