HLA (Human Leukocyte Antigens) are small proteins (called peptides) that are found on the surfaces of all cells in the body, and their function is to identify each cell as ‘self’ to the immune system. Foreign invaders (such as germs, viruses or parasites) don’t carry these antigens on their surfaces, which means the white blood cells of the immune system which search for abnormal or foreign cells recognise they don’t carry these markers, so they’re ‘non-self’ and will destroy them.
Everyone has two HLA-DQα markers on their cells, and these codes come from a specific area of DNA, with one marker coming from the mother, and one from the father. The DNA options for the HLA-DQα markers are limited and when the match between the mother and the fetus are too close, or identical, it can create significant problems with this ‘self’/’non-self’ recognition system.
Everyone (except identical twins) have differences in their DNA, and most of the time it really doesn’t matter too much whether certain sections of the DNA match very closely or exactly, but because the HLA-DQα section of DNA relates to the ‘recognition’ system of the immune response it really does matter when it comes to pregnancy. This is because in most conceptions the differences in the parents HLA-DQα means that the HLA-DQα on the placenta is different from the mother’s and the mother’s immune system is then able to recognise the pregnancy as a ‘foreign invader’ and starts a normal aggressive (Th1) response (see the immune system) to the pregnancy, which is followed by the mother’s suppressive (Th2) response responding to the Th1 action and making a blocking antibody that attaches to the placenta, and this ‘cloaks’ the pregnancy from a full Th1 (aggressive) response that would terminate it.
So, in normal pregnancies the body initially recognises the pregnancy as a non-self ‘invader’, but the suppressive side then ‘hides’ it from a fully aggressive immune response, and with both sides of the immune system now activated it allows the mother’s body to make the many essential changes to the placenta, womb, blood system etc for it to adapt and support a successful pregnancy.
When the father’s HLA markers on the placenta are too similar (or identical) to the mother’s HLA markers it means a full Th2 cloaking response won’t be possible by the mother, and unless the parents share exact HLA markers there are different possible levels of HLA-match between the mother and child which affects the possible outcome of the pregnancy: