(Selected Tables and Figures referenced, but not present in this blog
can be found in their corresponding Science Version blogs)
Mother’s womb is an absolute germless or sterile environment. As such, the fetus does not need an immune system. In fact, if it did have an immune system, it would react to the mother’s cells as being “foreign.” So nature has provided a safety valve by suppressing mother’s immune system in the womb during the early stages of fetal development. But these genetically distinct fetal cells start crossing the placenta into Mom and can be detected by the 4th or 5th week of the pregnancy and those fetal cells will remain in Mom’s immune system for years thereafter. A little benefit of this phenomenon is enhancement of mother’s milk for the newborn baby. But the real benefit of this process called “microchimerism” is believed to be the reason that Mom’s immune system adapts to the fetal cells and doesn’t interpret the growing fetus as foreign.
During fetal development, the immune systems of the developing fetus and the Mom are precisely timed during the pregnancy to achieve the best outcomes for both. During the first 12 weeks of the pregnancy, Mom’s innate immune system becomes more aggressive than normal to establish a successful implantation of the embryo where mother’s immune cells are active and produce a unique (to pregnancy) immune response called “passive immunity.” This immunity provides an abundance of chemical and proteins like immunoglobiins IgG and IgM that help the fetus get fully established during implantation. After that, for about 15 weeks, Mom’s immune system pretty much shuts down to the womb and allows the fetal cells to grow. Meanwhile, the fetus is developing its own immune system. Kind of interesting to have two immune systems functioning in one body. But it works! And finally, after the 12 weeks, Mom starts generating an aggressive immune system as she nears delivery. This is when certain chemicals produced by Mom help with the labor response.
The majority of genes shared in the mother and child relationship originate, would you believe, from Mom’s gut, or her gastrointestinal track. It makes sense when you realize that she is transferring them to the newborn during breast feeding and physical contact with the baby. The gut is part of a rather complex system of microorganisms called the microbiome. It’s a fascinating and extensive feature of human biology which we’ll take up in depth in Blog #13 and revisit it frequently throughout most of the other blogs. This microbial transfer from mother to baby is likely the reason mothers are the primary providers of the majority of genetic information to their offspring. Another fascinating aspect of this transfer of fetal cells from mother to fetus and newborn is the fact that the DNA of the cells from a male or female baby are detectable in the mother, especially in her brain, for the rest of her life. They call this “pregnancy brain” and is believed to have lifelong effects, positively in protection to the woman, and negative like lifelong potential autoimmune effects in the mother.
The distinctions and superiority of the female immune system that we have mentioned numerous times by now begins even earlier than its evolutionary origins. That passive immunity we mentioned above, produces a transfer of maternally-derived antibodies like IgG that transferred and shared in fetal circulation prior to birth through the placenta and through mother’s milk following birth. This does increase the risk of disease exposure, it also allows the infant's immune system to begin to “learn” about the local disease environment and causes the fetus to begin to acquire specific immune defenses that will endure beyond the pregnancy when passive immunity is no longer operative. In a sense, passive immunity can be viewed as a transient process (as in the Lamarckian theory of inheritance we mentioned in Blog #6) of acquired characteristics passed on from mother sharing her knowledge of disease to provide a kind of a buffer period while the infant builds its own repertoire of defenses through a Darwinian evolutionary process. Thus, to minimize the potential for off-spring infection in this passive immunity process, natural selection has evolved an increased antibody production in females.
After birth, the immune system continues to develop robustly until about age seven to eight when it reaches its strongest levels. At that point, general health, proper diet, and exercise will maintain a strong immune system for many years thereafter. But as we age, to be expected as with so many things, the system will begin to slow down a bit in response time and of course, it can be dangerously impaired with any form of immunocompromising disorders. But, by and large, the immune system continues to be our principal defense mechanism throughout life, notwithstanding increased risks of certain diseases (e.g., cancers) due to its slower response times with aging.
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