Blood Grouping is a method to tell what specific type of blood you have. What type you have depends on whether or not there are certain proteins, called antigens, on your red blood cells. ABO blood group system classifies four blood types: O, A, B and AB (in relation to erythrocyte agglutination with neither, anti-B, anti-A or both anti-A and anti-B antisera); donor blood must be compatible with the recipient's blood group, i.e. A can receive A and O blood; B can receive B and O blood; AB (universal recipient) can receive AB, A, B or O blood; O (universal donor) can receive only O blood.