Normally, T lymphocytes of our body can recognize foreign antigens, only when combined with specific surface molecules called major histocompatibility complex (MHC). All normal cells of the body express MHC class I molecules on their surface, whereas the MHC class II molecules are expressed only on antigen-presenting cells of the immune system. The trophoblast cells do not express MHC class I and class II molecules, thus, avoiding detection by the immune cells.