|MadSci Network: Medicine|
> Why don't blood group antibodies cross the placenta and kill the baby.
Hi - Antibodies against ABO antigens are almost always IgM antibodies. The IgM antibody consists of 5 antibody units connected together, in a star-like pattern. The protein is simply too large to cross the placenta. However, if a break in the placental-fetal barrier happens, the mother may generate a smaller immunoglobulin, IgG, against the ABO antigens on the fetal red cells. This Ig-type can cross the placenta. If early enough in the pregnancy, she may develop sufficient IgG antibody to affect the current pregnancy. The presence of the antibody can certainly affect future ones.
IgG antibodies against the Rh(D) antigen were once tbe most common cause of hemolytic disease of the newborn. In such cases the mother lacks the Rh(D) antigen on her red cells but the baby inherits the antigen from the father, and so has it on his or her red cells. The mother may generate antibody when exposed to the antigen from a prior pregnancy, or (far less commonly) from transfusion. Anti-Rh antibodies of the IgG subclass readily cross the placenta and can destroy the fetal red cells.
The introduction of RhoGam greatly decreased the number of HDN cases caused by Rh(D) incompatibility between the mother and the fetus. RhoGam consists of antibody against the Rh antigen. It is believed to act by rapidly binding to, and removing Rh-positive fetal red cells in the mother's circulation before she can generate antibody against the antigen.
-Lynn Bry, MD/PhD
Dept. Clinical Pathology
Brigham & Women's Hospital
Transfusion Medicine, J. McCullough, McGraw-Hill, 1998.
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