Blood is split into 4 major types: A, B, O, and AB. The type is based on a substance on the outside of the blood cell. Some people also have blood cells with a special protein. This protein is the Rh factor. If you have this protein on your blood cells, you are Rh-positive. If you do not have the protein, you are Rh-negative. Your blood type and Rh type are passed down to you from your parents. Rh incompatibility is also known as Rhesus disease. Rh incompatibility happens when a mother is Rh-negative and her baby is Rh-positive. This can happen when the father is Rh-positive. Rh-negative mothers may get a special drug while they are pregnant. This may help prevent their baby from having problems because they have a different blood type or Rh factor.