Blood types are classified based on the presence or absence of specific antigens on the surface of red blood cells. The main blood group systems are ABO and Rh. The ABO system includes four main types: A, B, AB, and O. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. The Rh factor indicates whether blood is positive (+) or negative (−). If the Rh antigen is present, the blood type is positive; if absent, it is negative. Knowing blood types is crucial for safe blood transfusions and organ transplants, as incompatible blood can cause serious reactions.