What are the four main human blood types?

The four main human blood types are A, B, AB, and O. This classification is based on the presence or absence of specific antigens on the surface of red blood cells. Blood type A has A antigens, blood type B has B antigens, blood type AB has both A and B antigens, and blood type O has neither A nor B antigens. Understanding these blood types is crucial for blood transfusions and organ transplants, as mixing incompatible blood types can lead to serious immune reactions.

The other options do not represent recognized blood types in human physiology. Choices that include letters such as X, Y, Z, and W, or P, Q, R, and S, or A1, B1, AB1, and O1 do not correspond to any established blood grouping system. The ABO blood group system is widely recognized and is the standard used in medical contexts.