Human blood is typed as either A, B, AB, or O, according to whether the red blood cells show the presence of certain antibodies. This classification system also includes a positive (+) or negative (–) suffix, dependent on whether the person’s blood has the Rh D antigen present. Both parents contribute to their offspring’s blood type, and so it is possible for a mother to have a different blood type than the child she carries. Blood type can be quickly determined by a simple blood sample test.