Blood type is determined by the specific antigens found in the blood. B blood type contains only the B antigen on the red cells, with an A antibody in the plasma. A third antigen, known as the Rh factor, determines whether the blood type is negative or positive.
An antigen is a marker found in the blood that initiates an immune response. The absence of the Rh antigen means a negative blood type, while the presence of the antigen indicates a positive blood type. Blood type is inherited, similar to eye and hair color. Antigens A and B are dominant genes, and type O is recessive. This means that if the A or B antigen is present, it determines the blood type over the presence of type O blood, which lacks both A and B antigens.