ABO blood groups are determined by the gene I (isoagglutinin). There are three alleles, $I^A, I^B$ and $I^O$ of this gene. Proteins produced by the $I^A$ and $I^B$ alleles are called $A$ antigen and $B$ antigen. People with blood group $A$ have the $A$ antigen on the surface of their RBCs, and antibodies against antigen $B$ in their plasma. Persons with blood group B have B antigen on their RBCs, and antibodies against $A$ antigen in their plasma. Individuals with $A B$ blood group have both antigen A and antigen $B$ on their RBCs, and no antibodies for either of the antigens in their plasma. Type $O$ individuals are without $A$ and $B$ antigens on their RBCs, but have antibodies for both these antigens in their plasma.