subsequent alloimmunization. they display dosage (react more strongly to homozygous cells versus heterozygous cells). transfusion reactions typically mild but may be serious, even fatal. although these usually occur immediately they may occur after a delay (up to 24 hours). these reactions are usually caused by anti-fya or anti-fyb. anti-fy3 may cause acute or delayed hemolytic transfusion reactions, but only rarely. anti-fy5 may also cause delayed hemolytic transfusion reactions. hemolytic disease of the fetus and newborn hemolytic disease of the fetus and newborn is typically mild but rarely may be serious. almost always due to anti-fya and rarely anti-fyb or fy3.