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Blood type is determined by the presence or absence of specific antigens on the surface of red blood cells. These antigens are glycoproteins or glycolipids that can trigger an immune response if they are foreign to the body. The most commonly recognized blood group systems are the ABO blood group system and the Rh factor.

In the ABO system, individuals may have type A, B, AB, or O blood, depending on whether they have A antigens, B antigens, both, or neither on their red blood cell surfaces. The Rh factor is determined by the presence of the D antigen; individuals who have this antigen are classified as Rh-positive, while those who do not are Rh-negative.

The presence of these antigens is crucial because it dictates compatibility when it comes to blood transfusions and organ transplants. Mismatches can lead to serious immune reactions, highlighting the importance of knowing a person's blood type. The correct identification of blood type based on these surface antigens is therefore fundamental in transfusion medicine and in ensuring patient safety.