Why does a person with type A blood cannot receice B blood?

A person with type A blood cannot receive B blood because their immune system would recognize the B antigens on the donor's red blood cells as foreign and attack them. This would cause a transfusion reaction, which could be life-threatening.

People with type A blood have anti-B antibodies in their plasma. These antibodies are produced by the immune system in response to the presence of foreign B antigens. If type B blood is transfused into a person with type A blood, the anti-B antibodies in the recipient's plasma will bind to the B antigens on the donor's red blood cells and cause them to agglutinate (clump together). This agglutination can block blood vessels and prevent oxygen from reaching vital organs, leading to serious complications such as hemolysis (the destruction of red blood cells), kidney failure, and even death.

Therefore, it is important for people with type A blood to only receive blood transfusions from donors who have type A or type O blood, as these blood types do not contain B antigens that would trigger an immune response.