Blood category is a term used to describe the four main types of blood that exist in humans. These four types are A, B, AB, and O, which are determined by the presence or absence of certain antigens on the surface of red blood cells. TYPE O is the most common TYPE, followed by A, B, and AB. Each TYPE is further divided into two subtypes, either Rh positive or Rh negative, depending on the presence of a particular antigen.