Which of the following best describes blood groups?

Blood groups are classifications based on the presence of specific antigens on the surface of red blood cells. These antigens are determined by genetics and play a crucial role in blood transfusions, immune response, and compatibility between blood donors and recipients. The most commonly known blood group systems are the ABO system and the Rh factor, which are primarily characterized by the types of antigens (A, B, AB, O, and Rh protein) found on individual red blood cells. Understanding blood groups is essential in medical settings to avoid serious complications that can arise from mismatched blood transfusions, as the immune system can recognize foreign antigens and initiate an immune response.

The other options do not accurately describe blood groups. Proteins and enzymes in blood do not define blood groups, though some blood proteins are involved in immunity and blood clotting. Hormones in the blood play roles in signaling and regulation of body functions but are unrelated to the classification of blood types.