In explaining how blood type has been used in an identification, the police lieutenant says, "You'll notice that Mrs. Collins' blood type is Rh positive". Rh is usually referred to as a "factor" or a "blood group system", not as a "blood type". In humans, blood types are limited to A, B, AB and O. A person with blood type "A positive" might also be said to be Rh positive, but a possible identification would made on the basis of the type, A, and not just the Rh factor.