Blood type is categorized into one of these eight groups: A-positive, A-negative, B-positive, B-negative, O-positive, O-negative, AB-positive ... for sneaking more exercise into your daily routine.

Blood type is categorized into one of these eight groups: A-positive, A-negative, B-positive, B-negative, O-positive, O-negative, AB-positive ... for sneaking more exercise into your daily routine.

These factors help define the eight most common blood types: A positive, A negative, B positive, B negative, O positive, O negative, AB positive and AB negative, says the Red Cross.

There are eight common blood types: A positive, A negative, B positive, B negative, O positive, O negative, AB positive and AB negative. But which is the rarest? Here's what to know.