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I have learnt that our immune system distinguishes self from non-self by using MHC. So a mother during pregnancy should also develop antibodies against the fetus as it is also a genetically different individual. Answering by saying that antibodies won't cross the placental barrier seems wrong as there is so much research going on this thing that such a simple explanation wouldn't suffice. Why doesn't the mother reject the baby?

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Perhaps an equally interesting question is why the immune system doesn't attack sperm. I've seen the answer before (but don't have a reference handy) and if I remember correctly, there's some special hard-coded marker, which is actually pretty scary. In any case, I think the mechanism is quite different from the one involved in your question. –  R.. May 20 '13 at 18:15
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In fact antibodies do cross the placental barrier: there is active transplacental transport of maternal IgG into the foetal circulation. –  Alan Boyd May 20 '13 at 21:32

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Awesome question! The immune system detects the presence of non-self using MHC and T cells and the lack of self using NK cells. The placenta first of all doesn't display MHC so T cells can't detect it, and expresses NK inhibitory receptors which stop NK cells killing it. Furthermore, the immune system isn't able to get to the foetus so it isn't able to activate against these antigens nor produce the antibodies which require T cells to be activated first, which then activate B cells. This is because the placenta is a syncitium, thus no spaces between cells for immune cells to slip through (a mechanism used by viruses). The placenta also secretes Neurokinin B, this makes it hard for the immune system to see the baby, and is a mechanism used by parasites. There are T reg cells which act to supress an immune response.

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thank u so much for answering . Your explanation seems quite plausible but I read in a book (Campbell Biology 9th edition) that still research is going on for finding explanation. So why is your answer not actual "answer" ? –  biogirl May 21 '13 at 15:31
    
I would imagine because it isn't the complete picture. I would imagine (but am not sure) that investigating the above features doesn't sufficiently depress the immune system preventing rejection of the foetus. –  AndroidPenguin May 21 '13 at 17:48
    
ok.Thanks once again. –  biogirl May 22 '13 at 17:53

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