Blood groups are monitored for blood transfer.
A person with blood group 'A' has 'A' antigen on the RBC and anti-B antibodies in the plasma.
A person with 'B' blood group has 'B' antigen on the surface of the RBC and anti-A antibodies in the plasma.
A person with 'AB' blood group has both 'A' and 'B' antigens and no antibodies in the plasma.
A person with blood group 'O' has no antigens on the surface of the RBCs and has both anti-A and anti-B antibodies in the plasma.