The C blood group system is one of the blood group systems in humans, classified based on the presence of specific antigens on the surface of red blood cells. It includes two main antigens, C and c, which are part of the Rh blood group system. These antigens can trigger immune responses if a person receives incompatible blood during a transfusion. Individuals with the C antigen are classified as C positive, while those without it are C negative. The presence of these antigens is important for blood transfusions and organ transplants, as compatibility can prevent serious complications.