An individual's blood differs from that of the other due to types of substances on the red blood cells. These substances are called antigens. We also know that there are antibodies in our plasma. These antibodies do not react to our own tissues but react with antigens on the red blood cells of another person. Thus we can group people on the basis of their antigens and antibodies into four groups: A, B, O and AB. People whose blood is A have antigen A and carry antibody B, Group B people have antigen B and antibody A and Group O people don not have either antigen A or antigen B. but both antibodies are present.


When blood is given to the patient (blood transfusion), it will be dangerous if the recipient patient gets blood with antigen against which he has antibodies. This will clump the red blood cells, transfused from the donor; the person who gives blood. This is a serious reaction. Thus it is vitally important to know blood groups of both the donor and the recipient. If the recipient belongs to group A, he will not be able to receive blood from group B donors. So same blood groups can be transferred and received.