IGHG3 and fetal erythroblastosis: In certain cases where the antibody titer was above the critical point, and the newborn didn’t show signs of HDFN with a serious intensity as expected, determination of subclasses complied to a low concentration of IgG1 and/or IgG3, or the mother’s serum contained other subclasses of IgG that have no capacity of complement activation, or are less efficient mediators of macrophagys -induced clearance of red blood cells.