The Rh factor is a protein on the surface of red blood cells.
The Rhesus (Rh) factor is an inherited protein that coats the surface of red blood cells. Some people have it and some don’t, and this, in part, is what determines your blood type, along with the A/B/O blood group system. If your blood has the protein, you’re Rh-positive (i.e. B+ or O+). If your blood lacks the protein, you’re Rh-negative (i.e. B- or O-). It’s more common to be Rh-positive, but being Rh-negative is not a health problem or an illness. However, it can affect a pregnancy if the mother is Rh-negative and the baby is Rh-positive.