Received January 20, 2010
Accepted April 15, 2010

The fetus is a tolerated allograft and that occurs natu- rally in pregnant Eomen. Half of the genes inherited from the father should be expressed and are allogenic to the mother. During the process of human embryo implanta- tion, both the embryo and the endometrium must stimu- late adaptations in the maternal immune system for the establishment of a viable pregnancy. The trophoblast is the outer layer of the placenta, the interface betEe- en maternal and fetal tissue, and does not express MHC Class I and II classic, Ehich makes it resistant to immune recognition mediated by T cells of the maternal immune system, but vulnerable to action of natural killer cells. The expression of MHC Class I non classic, human leukocyte antigen G (HLA-G) and human leukocyte antigen E (HLA- E) could be responsible for protecting the trophoblast from attack by natural killer cells. Interactions betEeen HLA-G, HLA-E and inhibitory receptors of NK cells result in a negative regulation of maternal immune response, helping to maintain the fetus semi-allogenic during preg- nancy. This revieE covers the main features of the mole- cule HLA-E, focusing on the participation of the molecule in the maternal-fetal interface, and hoE their actions could ensure the success of pregnancy. It is based on literature search of the last nine years (2000-2009), the databases OVID - Medline, Science Direct and Lilacs, and Pubmed.