Red Blood Cells (RBC) carry proteins on their surfaces in addition to the "A" and "B" antigens. The Rhesus factor, or Rh, is a protein carried by some individuals. If a person's RBCs carry the Rh factor, they are consider to be "Rh positive (Rh+)". If not, they are " Rh negative (Rh-)". Rh is a domain trait so only one allele is needed to be Rh+.