The Student Room Group

Agglutination

Why wouldn't you get aggultination of the blood if the maternal blood is blood group A and the foetal blood is blood group B?

Because in the maternal plasma there are anti-B antibodies and so surely when the blood crosses during birth there would be agglutination, yes?

But this isn't the case...

However, with Rhesus groups... if the mother is Rhesus negative and her first baby is Rhesus positive it causes antibodies against the Rhesus antigen on the red blood cell, but by the time this happens the baby is long gone...

However, in any other babies that are Rh+, then the mother would rapidly produce antibodies against the Rhesus group, which leads to aggultination. This is haemolytic disease of the newborn (HND).

Any ideas?
Antibodies to the ABO blood system are mostly IgM, so do not cross the placenta (as opposed to IgG for the rhesus system).

Interestingly a Rh- and O mother will not develop anti-Rh antibodies to a Rh+ foetus if it is also not O, as any blood cells that do cross are destroyed by anti-A/B antibodies before sensitisation. (This would clearly be the same with a A- mother and B+/AB+ foetus and so on)