Blood classification is a system used to categorize blood based on the presence or absence of certain antigens and antibodies. Blood TYPE is determined by the presence of two antigens, A and B, and the presence or absence of the Rh factor. TYPE A blood has the A antigen and may or may not have the Rh factor. TYPE B blood has the B antigen and may or may not have the Rh factor. TYPE AB blood has both the A and B antigens and may or may not have the Rh factor. TYPE O blood has neither the A nor B antigens and may or may not have the Rh factor.