Blood Types and Transfusion CompatibilitiesThe membranes of red blood cells contain certain markers, or antigens, which may be of different types. Furthermore, there are several major antigenic groups. The most important of these is the so-called ABO antigen group, which is based on the presence or absence of two major antigens, referred to as A and B. Any particular red blood cell can contain only one of four possible combinations of antigens from this group: A only, B only, A and B, or neither. A person whose erythrocytes contain only antigen A has type A blood; likewise, if only antigen B is present, an individualâ€™s blood type is B. If both A and B antigens are present, a person is said to have type AB blood, while type O refers to the absence of both antigens. Another blood group is referred to as the Rh group. Individuals whose red blood cells express the Rh factor (antigen) are Rh+, while those who lack it are Rh-. This is what is meant when a personâ€™s blood type is expressed, for example, as O positive: their red blood cells do not contain antigens A or B (from the ABO group), but do contain the Rh antigen.