The big picture: Blood type is determined by the presence or absence of specific antigens (proteins) on the surface of red blood cells (RBCs). The most widely used system for classifying blood is the ABO system, which categorizes blood into four main types:

Type A: Red blood cells have A antigens on their surface, and the plasma contains anti-B antibodies. This means type A blood will react against type B antigens.
Type B: Red blood cells have B antigens, and the plasma contains anti-A antibodies. This type reacts against type A antigens.
Type AB: Red blood cells have both A and B antigens, with no anti-A or anti-B antibodies in the plasma. This makes it the universal recipient because individuals with type AB blood can receive blood from any ABO type without a reaction.
Type O: Red blood cells lack both A and B antigens but have both anti-A and anti-B antibodies in the plasma. Type O negative is the universal donor because it can be safely transfused into anyone without causing an immune response.

Blood Transfusions: Matching donor and recipient blood types is crucial. If a patient receives blood with incompatible antigens, their immune system may attack the transfused blood cells, leading to potentially severe complications like hemolytic reactions, shock, or even organ failure.

Organ Transplants: ABO compatibility is essential for reducing the risk of organ rejection. Organ recipients must often receive organs from donors with a matching blood type to minimize immune response and improve transplant success.