G is a combination antigen in the Rh Blood Group System found on red cells containing either D or C antigens. Given the high frequency of D antigen expression (85% of whites, 92% of blacks, 99% of asians), it shouldn’t be surprising that the vast majority of people express G, even given the lesser expression of the C antigen. The main issue with G is that individuals can form an antibody to G that will react against either D or C positive cells and can’t be separated into two different antibodies. This antibody is usually not a big deal in the transfusion service (even though ruling out separate anti-D and anti-C will usually require some work such as elution studies that may result in a reference lab referral to separate the specificities). When transfusing someone with anti-G, off-the-shelf D-negative blood will be compatible in the vast majority of cases, since almost all D-negative blood is also C-negative (see the discussion of the Rh blood group elsewhere on this site. However, the distinction between anti-G and anti-D plus anti-C is crucial in pregnant, D-negative females. If a D-negative mom has anti-G, she does still need Rh prophylaxis in the form of Rh Immune Globulin. If, instead, she has anti-D plus anti-C, Rh prophylaxis is not indicated, since she is already immunized.
I wrote much more about G in my blog post, “So You Want to be a G-whiz!”
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