The surface of red blood cells contains markers that the immune system can recognize-one of these markers is the Rh factor (Rh [D] antigen). A person whose blood contains the Rh factor is Rh-positive (D); a person whose blood does not contain the Rh factor is Rh-negative (d).
Blood type contains information about the presence (+) or absence (-) of the Rh factor. Blood type is indicated by a letter (A, B, or O) and a symbol (+ or -). If a person has B negative (B-) blood, for example, that person is Rh-negative. A person with B positive blood (B+) is Rh-positive.
WebMD Medical Reference from Healthwise