This question has been partially elaborated due to fact that immune cells are never recruited to the site of embryo and therefore cannot harm the developing fetus. An interesting study you find it here:

P. Nancy, E. Tagliani, C.-S. Tay, P. Asp, D. E. Levy, A. Erlebacher. Chemokine Gene Silencing in Decidual Stromal Cells Limits T Cell Access to the Maternal-Fetal Interface. Science, 2012; 336 (6086): 1317 DOI: 10.1126/science.1220030

Dear Zahraa,

This article (recommended by Trim Lajqi) focused on mice chemokine system of feto-placental unit. On the other hand, there are some important check-point about maternal tolerance to feto-placental unit :

- Fetoplacental blood barrier (this barrier is not absolute) inhibits immune recognition .

-Trophoblast have no classical MHC molecules for antigen presentation to T cells (T cell unresponsiveness) .

-Decidua have big amounts large granular cells (NK). MHC negative cells are natural targets of NK cells. But, trophoblast have non-classical MHC molecules to inhibit NK cells.

-Placenta and fetus produce regulatory hormones, cytokines and peptides to regulate immune system.

As a conclusion Maternal-fetal interface is an immune privilege site and it is deviates maternal immunity.

Best regards

Ishak

Thank you very much dr.Trim Lajqi

Thank you very much dr.Ishak Ozel Tekin

Thank you so much Noor Mahdi

https://med.nyu.edu/pathology/research/featured-research-archives/maternalfetal-immune-interactions-why-doesnt-mother-reject

I am grateful to you dr. Anwar A. Abdulla

Take a look at the following RG links.

Article A balancing act: mechanisms by which the fetus avoids reject...

Article Maternal Acceptance of the Fetus: True Human Tolerance

Thanks so much dr.Arvind Singh for your kindful contribution and reply