ANTI-G

The G antigen is present on all RBCs that are D and/or C positive
            rare exception is the  rG cell
                       D and C negative but G positive

Why do we care about the G antigen and its antibody?
           When someone has anti-G it looks like they have antibodies to both D and C antigen
           Generally, you'd transfuse these patients with D negative and C negative blood which so happens to be G negative. So why do we care? If an obstetric patient has anti-G as opposed to anti-D + anti-C they do not have as severe Hemolytic Disease of the Newborn (HDN), but could make anti-D that would cause a severe form of HDN, so RhIg would be indicated.

       How do you know if it is anti-G or anti-D + anti-C?
                  Most reference labs will perform adsorption and elution studies to try to differentiate the two.
                        You'd utilize a D-C+G+ cell and incubate it with the patient's plasma. If anti-G is only present it will be removed from the plasma onto the cell, so when you elute the antibody you will find your eluate will react with both D+C- and D-C+ cells.
                                                 If it is anti-D + anti-C, your eluate will only react with the D-C+ cells.