Antigen A and antigen B are the antigens present on the surface of RBC's in people with different blood groups. People with blood group A have RBC's with antigen A on their surface, people with blood group B have RBC's with antigen B on their surface, people with blood group AB have both the antigens A and B on the surface of RBC's and people with blood group O have RBC's with no antigen on their surface.