Blood types define categories of people depending on the variety of antigens and antibodies which are present in their blood. They are classified according to a major ABO system, depending on the presence or absence of these proteins. There are three types of antigens A, B, and AB; A having only the A antigen on the surface of red blood cells (and anti-B antibodies), the B with only the Bs (and anti-As), and the AB having the 2 types of antigens (but no antibodies). Group O meanwhile, is characterized by the absence of these two types of antigens (but the presence of two types of antibodies). Another category, rhesus Rh + or Rh - is added. This factor depends on the presence or absence of another antigen, + is added to the letter if this protein is present in the blood , - in the case of its absence. Other subtypes are also described, which are less common. The importance of knowing one's blood type comes from the fact that in case of need for transfusion, the blood received may not be the same for different blood types because it must not be put into contact with the antibodies and antigens of the same type or it may cause coagulation of the blood. The people in the group O- are "universal donors" but can only receive blood from people of the same type. People in the AB+ group are universal receivers and can not give blood within the AB+ group.