~erythroblastosis fetalis~

Hemolytic disease of the newborn, also known as Hemolytic disease of the fetus and newborn, HDN, HDFN, or Erythroblastosis fetalis, is an alloimmune condition that develops in a fetus, when the IgG molecules (one of the five main types of the antibodies) that have been produced by the mother and have passed through the placenta include ones which attack the red blood cells in the fetal circulation. The red cells are broken down and the fetus can develop reticulocytosis and anaemia. This fetal disease ranges from mild to very severe, and fetal death from heart failure (hydrops fetalis) can occur. When the disease is moderate or severe, many erythroblasts are present in the foetal blood and so these forms of the disease can be called erythroblastosis fetalis (or erythroblastosis foetalis).

Antibodies are produced when the body is exposed to an antigen foreign to the make-up of the body. If a mother is exposed to a foreign antigen and produces IgG (as opposed to IgM which does not cross the placenta), the IgG will target the antigen, if present in the fetus, and may affect it in utero and persist after delivery. The three most common models in which a woman becomes sensitized toward (i.e., produces IgG antibodies against) a particular blood type are:
Fetal-maternal hemorrhage can occur due to trauma, abortion, childbirth, ruptures in the placenta during pregnancy, or medical procedures carried out during pregnancy that breach the uterine wall. In subsequent pregnancies, if there is a similar incompatibility in the fetus, these antibodies are then able to cross the placenta into the fetal bloodstream to attach to the red blood cells and cause hemolysis. In other words, if a mother has anti-RhD (D being the major Rhesus antigen) IgG antibodies as a result of previously carrying a RhD-positive fetus, this antibody will only affect a fetus with RhD-positive blood.
The woman may receive a therapeutic blood transfusion with an incompatible blood type. ABO blood group system and the D antigen of the Rhesus blood group system typing are routine prior to transfusion. Suggestions have been made that women of child bearing age or young girls should not be given a transfusion with Rhc-positive blood or Kell1-positive blood to avoid possible sensitization, but this would strain the resources of blood transfusion services, and it is currently considered uneconomical to screen for these blood groups. HDFN can also be caused by antibodies to a variety of other blood group system antigens, but Kell and Rh are the most frequently encountered.
The third sensitization model can occur in women of blood type O. The immune response to A and B antigens, that are widespread in the environment, usually leads to the production of IgM anti-A and IgM anti-B antibodies early in life. On rare occasions, IgG antibodies are produced. In contrast, Rhesus antibodies are generally not produced from exposure to environmental antigens.