Page 1137 - Veterinary Immunology, 10th Edition

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FIG. 34.8 Some of the immunosuppressive factors and processes
that prevent rejection of the fetus by the mother's immune system.

First, no polymorphic (classical) MHC molecules are expressed
on preimplantation embryos or oocytes. Likewise, polymorphic
MHC class Ia or class II molecules are not expressed on the cell
layer of the trophoblast in contact with maternal tissues. Cytokines

that usually enhance MHC expression, such as IFN-γ, have no
effect on trophoblast cells. The absence of classical MHC molecules
should permit trophoblast cells to avoid destruction by cytotoxic T

cells but make them susceptible to attack by NK cells. This is
prevented, however, by expression of the nonpolymorphic MHC
class Ib molecules HLA-C, HLA-G, and HLA-E. These molecules
inhibit NK cell cytotoxicity while the NK cells control invasion of
the uterine wall by the trophoblast. Thus there is a balance between

MHC class Ib expression on the trophoblast and NK cells in the
uterine wall that together regulates trophoblast growth and
invasion. Bovine classical MHC class I molecules are not expressed

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