After birth, two types of antibodies i.e., anti-A & anti- antibodies appear in the blood of individuals. These antibodies are present according to the absence of the corresponding antigen, in persons with blood group A, antigen A is present and antigen B is absent. So, their blood serum will contain anti-B antibodies. In persons with blood group B, antigen B is present and antigen A is absent. So, their blood serum will contain an anti-A antibody. In persons with blood group AB, antigens A & B are present i.e., neither is absent. So, their blood serum will contain no antibody. In persons with blood group O, neither antigen A nor antigen B is present i.e., both are absent. So, their blood serum will contain both anti-A& anti-B, antibodies.