We recently have discovered a Bg antibody in a patient who is about 30 wks pregnant. I'm having trouble finding much literature on this so that I can communicate with her caregivers what exactly it is and where it comes from. She was transfused 17 yrs ago and says this is her first pregancy. I know it is not generally considered clinically significant.

Bg antibodies are antibodies directed against HLA Class I antigens.

These antigens are expressed quite strongly on virtually all nucleated cells, but are poorly expressed on red cells.  This is purely down to the number of antigens sites on the various cells.  For example, a T lymphocyte will express some 100, 000 such antigens on their surface, whereas a red cell will only express from 40 to 500 such antigens.

It was originally thought that Bg antigens on red cells were adsorbed onto the red cell surface from the plasma, but it now seems that these antigens may be intrinsic, having been formed during the time when the erythrocyte precursors actually had a nucleus, but, that notwithstanding, they can easy be removed from the red cell by chloroquine treatment.

Bg^a is analogous with HLA-B7, Bg^b is analogous with HLA-B17 and Bg^c with HLA-A28, but there may be cross-reactivity with other HLA antigens.

Bg antibodies are very common in pregnancy, having been stimulated by the foetal HLA antigens, but have never been implicated in clinically significant HDFN, so neither you, nor the expectant mother need to worry (they are "nuisance" antibodies).

For more information, try Geoff Daniels, Human Blood Groups, third edition, 2013, Blackwell Publishing Ltd. Chapter 32 (pages 512 - 514) - so there is not a lot to read!