Blood group antigens are specific molecules found on the surface of red blood cells that determine an individual's blood type. The most well-known blood group systems are the ABO system and the Rh system. These antigens can trigger immune responses if blood from different groups is mixed, making compatibility crucial for blood transfusions. Each blood type is defined by the presence or absence of certain antigens. For example, type A blood has A antigens, type B has B antigens, type AB has both, and type O has none. Understanding blood group antigens is essential for safe medical practices, including organ transplants and pregnancy management.