[Scott Fairclough]

Scott Fairclough  asked the Naked Scientists:

Hi Chris,

I have a question relating to pregnancy and I've never been able to  
find a good answer for this, please can you help.

My question is why doesn't the mothers own immune system attack a  
developing baby during pregnancy since its DNA will be different from  
the mother's (50% the father's) why doesn't the mothers immune system simply see it as foreign and respond as such ?

Regards

Scott from Manchester, UK

What do you think?

[Uhura101]

There are a number of reasons why a baby is not attacked by the mother's immune system. Cells of the mother's immune system cannot cross the placenta into the baby. Only IgG anibodies can cross the placenta into the babies circulation, all other antibody classes are too big. The IgG which does cross the placenta is what gives the baby immunity during its first six months before its own immune system activates. Sometimes these IgG antibodies from the mother do attack the father's antigens in the baby's tissues. This is most commonly seen where a mother's antibodies attack a baby's red cells. This can and does cause intra-uterine death (called Bart's Hydrops). The reason this does not happen more often is that many of the antigens in the baby's tissues are not fully developed and the mother's antibodies do not readily bind to them.

David
Sydney, Australia