Antigens are surface proteins found on red blood cells. Type O negative blood is the only blood type with no antigens.
Blood type A has an A antigen, while blood type B has a B antigen. Blood type AB has both A and B antigens. Blood type O has neither. But assigning a letter is just the first step in determining your specific blood type.
In addition to the aforementioned A and B antigens, blood is classified based by the presence of the Rh factor. Blood types with the Rh factor are considered Rh positive, and those without it are considered Rh negative. When we account for A and B antigens as well as the Rh factor, we get the eight most common blood types: A+, A-, B+, B-, O+, O-, AB+ and AB-.
Our immune system works to protect us from such invaders as bacteria and viruses. Our immune system recognizes the antigens present in our own blood type, but it may go into defense mode if it detects antigens from a different blood type. If mismatched blood is given during a transfusion, the body may fight back, potentially endangering the life of the patient.