Final answer:
Blood types are determined by the presence or absence of A and B antigens on the surface of red blood cells, which define the ABO blood group system. These antigens lead to the designation of blood types A, B, AB, and O. The Rh factor, indicated by '+' or '-', is another antigen system separate from ABO.
Step-by-step explanation:
Human blood types in the ABO blood group are determined by the presence or absence of antigens on the surface of red blood cells. These antigens are specific molecular structures that can be either proteins or glycolipids. People can have A antigens, B antigens, both A and B antigens (AB), or neither (O), on their red blood cells. This genetic trait is responsible for the four basic ABO blood types. The Rh system is another antigen system on red blood cells, where the '+' or '-' in blood types (e.g., A positive or A negative) signifies the presence or absence of the Rh factor, which is separate from the ABO blood type antigens.
When incompatible blood types are mixed, such as during a blood transfusion, the immune system may recognize the foreign antigens and produce antibodies, leading to a potentially dangerous reaction known as agglutination. Therefore, understanding an individual's blood type is crucial in clinical settings to ensure compatibility for blood transfusions and organ transplantation.