Blood type tests are done before a person receives a blood transfusion. It is also used to check a pregnant woman's blood type. Human blood is classified, or typed, according to the presence or absence of certain markers (called antigens) on the surface of red blood cells. Blood type may also be done to see if two people are likely to be blood relatives (for instance, to help establish paternity when it is in question). The most important antigens are blood group antigens (ABO) and the Rh antigen. Therefore, the two most common blood type tests are the ABO and Rh tests. Although there are other ways to type blood, these two tests are the most common.