Blood types are classified based on what feature?

Blood types are classified based on the presence or absence of specific antigens on the surface of red blood cells. These antigens are proteins or carbohydrates that can provoke an immune response if they are foreign to the body. The most commonly known blood group systems are the ABO and Rh systems.

In the ABO system, for example, individuals with type A blood have A antigens on their red blood cells, those with type B have B antigens, type AB has both A and B antigens, and type O has neither. The Rh system adds another layer of classification based on the presence (+) or absence (−) of the Rh factor antigen. This classification is crucial for blood transfusions, organ transplants, and pregnancy, as incompatible blood types can trigger severe immune reactions.

Other options, such as the shape of red blood cells, while significant in conditions like sickle cell anemia, do not determine the blood type classification. The age of the blood cells is not relevant to blood type and the concentration of lymphocytes pertains to immune function rather than blood classification. Thus, the defining feature for blood type classification is indeed the presence or absence of specific antigens.