Human blood is grouped by two systems. The first grouping is the A, B, and O system. The second most important grouping system is Rhesus factor, or Rh(D). Rhesus (Rh) factor is a protein located on the surface of red blood cells and determines if the donor’s blood has a positive or negative Rh factor. If a donor’s red blood cells have this protein, he is Rh-positive. If his blood does not have the protein, he is Rh-negative. Most people have the Rhesus factor and, therefore, have an Rh-positive blood type. Having Rh-negative blood is not an illness and usually has no effect on your health. However, it can affect an expectant mother’s baby during pregnancy. If you have an Rh-negative blood type, you could help save babies!