HDFN represents a breach of immune privilege for the fetus or some other form of impairment of the immune tolerance in pregnancy. Various types of HDFN are classified by which alloantigen provokes the response. The types include ABO, anti-RhD, anti-RhE, anti-Rhc, anti-Rhe, anti-RhC, multiantigen combinations, and anti-Kell. Although global prevalence studies of the differential contribution of those types are lacking, regional population studies have shown the anti-RhD type to be the most common cause of HDFN, followed by anti-RhE, anti-RhC, and anti-Rhc.