|MadSci Network: Immunology|
An excellent question, and one that has baffled scientists for years. Firstly in the assisted fertility treatments involved in surrogacy an embryo (not a ovum) would be introduced into the uterus of the surrogate mother. An embryo used in a surrogacy situation is no different to an embryo produced by natural fertilization. It has come from sperm and ovum. That is to say that it is genetically, and therefore immunologically and phenotypically, a different human being. The question is, therefore, why is ANY fetus not rejected, through antigen/antibody interaction? Antigens first appear on the embryo shortly after implantation (about 8-10 days after fertilization) and are therefore perfectly capable to elicit immune responses and be rejected. The complete answer still eludes science to this day. Here are a few possible explanations 1. Fetal and maternal blood systems are separated by the placenta (immunologically this is fetal tissue and therefore foreign). At the border between the mother and fetus the placental cells (trophoblasts) do not have “normal” antigens on their surface but special ones that might interact differently – this is still being researched. 2. A special population of natural killer cells that interact with these antigens and regulate the maternal (mothers) immunology by cell signaling. 3. The immunology of the mother is dampened possibly by the high levels of pregnancy hormones. 4. The separated system barriers also act like a filter and only some maternal antibodies (IgG’s) are allowed to pass through. It is believed that the fetus “mops up” these few antibodies before they can build up to cause problems. An exception to this is the rhesus factor of red blood cells, if the mother is rhesus –ve and the fetus is rhesus +ve immunological rejection can and does occur!
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