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In an attempt to expand on Naina's answer the A,B,O blood time is determined by the presence or absence of specific carbohydrates (referred to above as surface antigens) on the erythrocytes, or red blood cells that are recognized by the individuals immune system. For instance, let us say that Type A is characterized by circle carbohydrates and Type B is represented by triangles. The immune system of a type A patient would recognize the triangle antigens as foreign and therefore elicit an immune/inflammatory response. For this reason Type B blood is incompatible for a Type A patient.
Furthermore, AB blood has the presence of both surface antigens so there is no immune response upon transfusion of Type A, B, AB or O blood. O in turn is completely devoid of these surface antigens and is therefore the universal donor. The Rh factor is an additional category represented by + or -. O- is truly the universal donor when you take into account Rh factor