In the context of blood types, what does the ABO system classify?

The ABO system classifies blood types based on the presence or absence of specific antigens on the surface of red blood cells. Specifically, this system identifies two main antigens, A and B. If a person's red blood cells have the A antigen, they are classified as type A blood. If they have the B antigen, they are type B blood. If both antigens are present, the individual has type AB blood, and if neither is present, they have type O blood.

This classification is vital for blood transfusions, organ transplants, and understanding immune responses, as the presence of these antigens can trigger immune reactions in individuals who receive incompatible blood types. Understanding the role of these antigens is essential for safe and effective medical practices involving blood. Other options, such as the number of red blood cells, blood viscosity, or plasma proteins, do not relate directly to the ABO classification system and instead pertain to different aspects of blood composition and function.