APA Dictionary of Psychology

any of at least eight different antigens, each determined genetically, that may be attached to the surface of an individual’s red blood cells (the name derives from the rhesus monkey used in early studies of the factor). A person whose blood cells carry an Rh factor is said to be Rh-positive. One whose blood cells lack an Rh factor is Rh-negative. Some 99% of African Americans, Native Americans, and Asian Americans and 85% of European Americans are Rh-positive. See also Rh blood-group incompatibility.