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IgM is the first immunoglobulin expressed in the human fetus (around 20 weeks) [46] and phylogenetically the earliest antibody to develop. [47] IgM antibodies appear early in the course of an infection and usually reappear, to a lesser extent, after further exposure. IgM antibodies do not pass across the human placenta (only isotype IgG). [48]
Hemolytic disease of the fetus and newborn (HDN) is a condition where the passage of maternal antibodies results in the hemolysis of fetal/neonatal red cells. The antibodies can be naturally occurring such as anti-A, and anti-B, or immune antibodies developed following a sensitizing event. [ 38 ]
Mothers who are negative for the Kell 1 antigen develop antibodies after being exposed to red blood cells that are positive for Kell 1.Over half of the cases of hemolytic disease of the newborn owing the anti-Kell antibodies are caused by multiple blood transfusions, with the remainder due to a previous pregnancy with a Kell 1 positive baby.
The immune response to A and B antigens, which are widespread in the environment, usually leads to the production of IgM or IgG anti-A and anti-B antibodies early in life. Women of blood type O are more prone than women of types A and B to making IgG anti-A and anti-B antibodies, and these IgG antibodies are able to cross the placenta.
Other examples of insufficient immune tolerance in pregnancy are Rh disease and pre-eclampsia: Rh disease is caused by the mother producing antibodies (including IgG antibodies) against the Rhesus D antigen on their baby's red blood cells. It occurs if the mother is Rh negative and the baby is Rh positive, and a small amount of Rh positive ...
IgG antibodies are most reactive at 37°C. IgM antibodies are easily detected in saline at room temperature as IgM antibodies are able to bridge between RBC's owing to their large size, efficiently creating what is seen as agglutination. IgG antibodies are smaller and require assistance to bridge well enough to form a visual agglutination ...
Routine antenatal antibody screening blood tests (indirect Coombs test) do not screen for ABO HDN. [citation needed] If IgG anti-A or IgG anti-B antibodies are found in the pregnant woman's blood, they are not reported with the test results, because they do not correlate well with ABO HDN.
The term "Rh Disease" is commonly used to refer to HDFN due to anti-D antibodies, and prior to the discovery of anti-Rh o (D) immune globulin, it was the most common type of HDFN. The disease ranges from mild to severe, and occurs in the second or subsequent pregnancies of Rh-D negative women when the biologic father is Rh-D positive.