What is meant by the Rh (rhesus factor)?

Most people have an inherited antigen on their red blood cells known as Rh, or D antigen. When the D antigen is present, a person’s blood is designated Rh positive. When D antigen is missing, the blood group is designated Rh negative. In general, Rh negative blood is given to Rh negative patients, and Rh positive blood or Rh negative blood may be given to Rh positive patients.