Rhesus factor (Rh factor) is a group of antigens that may or may not be present on the surface of red blood cells (erythrocytes). Rhesus factor forms the basis of the rhesus blood group system. Most people (about 85%) have the rhesus factor, i.e., they are Rh-positive. People who lack the factor are said to be Rh-negative.
See also blood groups.