Human blood is classified according to the presence or absence of specific markers, called antigens, on the surface of red blood cells. The most commonly encountered blood group system is the ABO system, which classifies blood based on the presence or absence of two antigens, A and B. There are four blood types in this grouping, A, B, AB, and O. Type AB indicates the presence of both antigens and O the absence of both. What do the symbols + and – represent in the classification of blood? Discuss