H Blood Group
Number of Antigens: 1
Transfusion Reaction: Yes—can cause an acute hemolytic transfusion reaction
Hemolytic Disease of the Newborn: Yes
HDN could arise in mothers with the Bombay phenotype (Oh, h/h).
"In theory, the maternal production of anti-H during pregnancy could cause hemolytic disease in a fetus who did not inherit the mother's Bombay phenotype. In practice, cases of HDN caused in this way have not been described, possibly because of the rarity of the Bombay phenotype."
Update - 2013 saw a case of HDN caused by anti-H because the infant did not inherit the mother's Bombay phenotype.