The antibodies are present in human blood without prior exposure to the antigen, which means that people with a particular ABO antigen carry the antibody against the other antigen in their serum. A person with 'B' blood group carries 'B' antigen on their RBCs and only anti- A antibodies in their serum. A person with 'A' blood group carries 'A' antigen on their RBCs and only anti- B antibodies in their serum. Since 'AB' blood group has both 'A' and 'B' antigen on their RBC, both of antibodies are absent in the serum. Since type O persons do not have either antigen on their RBCs, so it carries both anti- A and anti- B antibodies in the serum.