![]() ![]() The antibodies transferred across the placenta are mostly IgG ( see “Adaptive immune system”). Because of this late transfer, babies born prematurely tend to have lower levels of antibodies circulating in their blood and are, therefore, more susceptible to infections than full-term newborns. However, most of the antibodies cross the placenta late in pregnancy during the third trimester, so they will be plentiful at the time of birth. Via the placenta - Antibodies generated by the mom’s immune system begin to cross the placenta by the 13 weeks of gestation.While these things are happening, the maternal immune system steps up to help in two ways: In this regard, it is “antigenically inexperienced,” so it needs to gain experience. Second, the fetus’s immune system has not previously responded to any pathogens. First, as mentioned above, in the womb, suppression factors kept the fetus’s immune system from competing with maternal immune responses, but it needs to immediately start working upon birth. In the period after birth, the newborn’s immune system has two immediate hurdles to clear. Transitioning from a sterile to a non-sterile environment These consequences become important once the baby leaves the sterility of the womb and encounters the bacteria-rich environment of the vaginal tract and the unsterile world in which we live. This exchange of cells from the fetus to the pregnant woman provides another possible explanation as to why a mother’s immune system does not reject the growing fetus.Īnd, while it is essential that the fetus grow in an environment in which it is not rejected, this is not without consequence for the developing immune system. The presence of genetically distinct cells in an individual, such as the fetal cells found in a mother, is called microchimerism. These cells can be detected in the mom between the fourth and fifth week of pregnancy, and they remain for years, even decades, after she has given birth. Second, fetal cells cross the placenta and circulate in the mother. First, because the fetal immune system does not need to function while in the womb, it can safely be suppressed. This means, the immune system must be able to distinguish between “self” and “non-self.”īabies are not genetically identical to their mothers, so why doesn’t a woman’s immune system attack this “non-self” entity and reject it as foreign tissue? Scientists have not completely figured this out yet, but they do have some important clues. If you have read the previous pages of this section or if you have thought about organ donation, one of the things you may realize is that our immune systems are designed to protect us from outside attacks. Likewise, B and T cells can be found in the intestine by about 16 weeks of gestation and by about 19 weeks of gestation, they are organized into specialized lymph nodes in the intestine, called Peyer’s patches. Macrophages can be found in the fetal intestine by 11 or 12 weeks of gestation, and quantities increase rapidly during the fourth and fifth months of pregnancy. This is important because one of the most intriguing aspects of the immune system relates to pregnant women (See “Two immune systems, one body” section below). Although these T cells are functional by the third or fourth month of pregnancy, the sterile environment of the womb does not require the fetal immune system to fend off potential pathogens. ![]() By the second or third month of pregnancy, some are already becoming T cells. The early progenitor cells travel through the blood into organs associated with the immune system, such as the liver, spleen and thymus. Babies born prematurely tend to have greater quantities of these unspecialized progenitor cells than full-term babies. Early in the pregnancy, these cells divide very rapidly, but as the fetus matures, they decrease in the speed with which they multiply and more of them become specialized cell types. These early immune system cells, called hematopoietic progenitor cells, have proteins on their surface that allow scientists to identify them as precursors of immune system cells. But even this early in development, some of the cells that are busy dividing and specializing will eventually become immune system cells. The fetus is only the size of a poppy seed. During the fourth week of pregnancy, a woman may just be finding out that she is going to have a baby. ![]()
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