Haemolytic disease of the newborn and maternal IgG
Haemolytic disease of the newborn (breakdown of the baby's red blood cells) happens when maternal IgG, an antibody class, crosses the placenta and attacks them: a type II hypersensitivity.
Remember
Haemolytic disease of the newborn is a type II hypersensitivity caused by maternal IgG, because IgG is the antibody class that crosses the placenta, whereas IgM causes type II transfusion reactions instead.
Type II hypersensitivity (an immune reaction in which antibodies target antigens, molecules the immune system recognises, found on the surface of cells) is mediated by IgG or IgM antibodies. Haemolytic disease of the newborn is one example, and it is IgG mediated.
The reason lies with the placenta (the organ that links mother and foetus), which IgG can cross and other antibody classes cannot. IgG (immunoglobulin G, an antibody class) can cross the placental membranes into the foetal circulation. IgM also mediates type II reactions, for example against mismatched donor blood in a transfusion, but IgM does not cross the placenta.
The mother lacks a foetal red cell antigen, so her immune system treats it as non-self and makes antibodies against it. Her IgG reaches the baby, binds the red cells and marks them for destruction by phagocytes (cells that engulf and destroy other cells). This destruction is haemolysis (breakdown of red blood cells). The baby becomes anaemic (too few red cells to carry oxygen) and jaundiced (yellow skin and eyes from a build-up of bilirubin, a waste product that the liver normally clears).
In the Rhesus form, an Rh negative mother (her red cells lack the RhD antigen) can make anti-D antibodies after contact with Rh positive blood, for example during an earlier pregnancy with an Rh positive baby. An anti-D injection is offered to an RhD negative mother carrying an RhD positive baby, to stop her developing antibodies.
In the ABO form, a group O mother carries natural antibodies against the A and B antigens (the markers on red cells that define the ABO blood groups), and these include IgG. A group A or B baby can therefore be affected through the same mechanism, and a first pregnancy can be affected.
Key facts
- Reaction type
- type II hypersensitivity, mediated by IgG or IgM antibodies against antigens on the surface of cells.
- Antibody
- maternal IgG, the antibody class that crosses the placenta into the foetal circulation.
- Result
- IgG coats the baby's red cells, causing haemolysis, anaemia and jaundice.
- Rhesus form
- the typical set-up is an RhD negative mother with antibodies and an RhD positive baby; prevention is an anti-D injection, offered to an RhD negative mother carrying an RhD positive baby, to stop her developing antibodies.
- ABO form
- the typical set-up is a group O mother with a group A or B baby; it is usually milder than Rhesus disease.
- IgM contrast
- IgM mediates haemolytic transfusion reactions but is not the class that crosses the placenta.
Exam traps
- IgM can mediate type II reactions, such as those against mismatched donor blood, but it is not the antibody class that crosses the placenta into the foetal circulation, so maternal IgM does not cause haemolytic disease of the newborn.
- Immune complex deposition (antibody stuck to free, soluble antigen that lodges in tissues) is type III hypersensitivity, so it does not explain haemolytic disease of the newborn, where antibodies target antigens fixed on the surface of red cells and the reaction is type II.
- Killing of red cells by T cells is a type IV mechanism, so it does not explain haemolytic disease of the newborn, which is a type II reaction caused by maternal IgG binding to foetal red cells.
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