Dom Spinella dspinella at
Mon Apr 13 15:21:09 EST 1998

> please, could anyone tell me how ABO antibodies(IgM) can cause damage
> to a fetus, if these kind of antibodies can not cross the placenta?
> thanks in advance.
> carlos

Well, mostly they don't.  Given the allele frequencies of the A, B, and
O alleles, a majority of all human pregnancies will involve an ABO
incompatibility between mother and fetus -- and yet hemolytic disease
due to ABO antibodies is quite rare.  In fact, such incompatibilities
are somewhat protective against Rh incompatibility -- which is also much
more common than the incidence of Rh-associated hemolytic disease (even
in the days before RhoGam). Existing ABO antibodies in the mother's
serum destroy fetal cells introduced into maternal circulation during
parturition before she can become immunized against Rh or other blood
group antigens on the fetal cells. 

As you suggest, the large size of the IgM antibodies that are typically
formed against ABO antigens preclude their crossing the placental
barrier.  However, it is possible in unusual cases that a mother can
mount an IgG response against these antigens (particularly when exposed
to bacterial homologs during pregnancy) -- although again, IgG responses
to T-independent carbohydrate antigens such as ABO are rare. 
Presumably, these antibodies, if they are formed, can cross the placenta
and do damage.

Dom Spinella

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