The blood group is determined by the type of antigen on the surface of the red blood cells. Blood grouping is a method of categorizing blood, usually referring to the classification of red blood cells, based on the presence or absence of some heritable antigenic material on the surface of red blood cells. Blood cells with antigen A on their surface are called blood type A, those with antigen B on their surface are called blood type B, those with antigens A and B on their surface are called blood type AB, and those without antigens A and B on their surface are called blood type O. Blood types are inherited from the parents. Blood type is genetically determined by the blood type of both parents, in accordance with Mendel’s law. The blood type test is performed by drawing blood from a vein. Fasting is not required for the blood test and blood can be drawn at any time of the day. For interpretation of specific indicators, consult a hematologist.