Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Rh Incompatibility ?
    ### Response:
    There are four major blood types: A, B, O, and AB. The types are based on substances on the surface of the blood cells. Another blood type is called Rh. Rh factor is a protein on red blood cells. Most people are Rh-positive; they have Rh factor. Rh-negative people don't have it.
    