How Everything Works on Instagram: "Blood types are determined by molecules called antigens that sit on the surface of red blood cells. These act as identification markers, helping the immune system recognize which cells belong in the body. The main system for classifying blood is the ABO system, which divides blood into four groups: A, B, AB, and O. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. Another key component is the Rh factor, which indicates whe