Why Doesn’t the Mother’s Immune System Reject a Baby?
Every pregnancy presents the immune system with a remarkable challenge. The developing fetus nestled in the mother’s uterus should be seen as “foreign” to the mother’s immune system, but its not. A pregnant person is able to promote growth and development of the fetus for nine months, without any immune suppressive treatments. This is in sharp contrast to organ transplantation where the immune system will rapidly reject the new organ if the patient is not on immune suppressive medications.
Is the Immune System Suppressed During Pregnancy?
For many years, scientists believed pregnancy worked by suppressing the immune system. Today, researchers recognize that pregnancy involves complex changes that help the immune system balance two important jobs: protecting both parent and fetus while still defending against infection. Suppressing the immune system is one part of this and could explain why fetal tissues are not rejected by the immune system. It could also help explain why some autoimmune disorders such as multiple sclerosis and rheumatoid arthritis often improve during pregnancy.
However, if the mother’s immune system is suppressed, how does it still fight infections and responds to vaccines? These inconsistencies indicate that the immunological changes during pregnancy are far more complex. A better understanding of how pregnancy immunologically works could improve pregnancy outcomes as well as shed light on a variety of other contexts including organ transplantation and autoimmunity.
Does the Immune System Cause Pregnancy Complications?
Pregnancy complications are more common than most people think. An estimated 1 in 10 babies are born too early. Furthermore, pregnancy complications such as pre-term birth, preeclampsia, a pregnancy complication characterized by high blood pressure and signs of organ dysfunction, and stillbirth, are consistently the leading cause of death in babies and reproductive age women.
Babies that survive these complications are at an especially high risk for developing chronic disorders such as cerebral palsy, blindness, and respiratory and metabolic problems. There are no effective cures for these conditions partly because our understanding of how pregnancy works remains limited.
Why Is Pregnancy So Difficult to Study?
Several considerations contribute to this lack of understanding. Pregnant women are generally restricted from most research studies. This is partly because researchers want to protect this vulnerable population from unnecessary risk. While the benefits and risk of any study need to be carefully considered, it may be helpful to change this thinking to protect pregnant women through research, rather than from research.
Without the clinical data from human studies, researchers have developed animal models of pregnancy that show remarkable similarity in the immunological changes occurring during pregnancy across species, including humans. Using information collected from studying these models, some researchers propose that abnormal immune responses at the maternal-fetal interface may contribute to complications such as preterm birth, preeclampsia, stillbirth, and miscarriage.
Can the Immune System Remember Previous Pregnancies?
One of the most intriguing discoveries in recent years is that the immune system may “remember” previous pregnancies.
Immunological memory is one of the defining features of the immune system. It is the reason vaccines work and why many people only get certain infections, such as chickenpox, once. After encountering a threat, specialized memory cells remain in the body and allow faster, more effective responses in the future. Researchers are now investigating whether a similar process occurs during pregnancy.
Studies in mice suggest that certain immune cells expand during pregnancy and develop properties that help promote tolerance of the fetus. Some of these cells appear to persist long after birth.
When another pregnancy occurs with the same father, these pregnancy-associated immune cells can accumulate more quickly. In animal studies, this has been associated with a lower risk of pregnancy complications.
Researchers have also observed a similar pattern in humans. Following a healthy pregnancy, the risk of complications such as preterm birth, preeclampsia, and stillbirth is often lower in later pregnancies. Some studies have found that these protective effects are reduced when subsequent pregnancies involve a different father, suggesting that the immune system may develop a form of memory related to fetal tissues.
Looking Ahead
Pregnancy challenges many traditional ideas about how the immune system works. Far from simply suppressing immunity, pregnancy requires the immune system to perform an extraordinary balancing act—protecting both parent and fetus while avoiding harmful immune attacks. Scientists are still uncovering how this process works, but each new discovery brings us closer to understanding one of biology’s most remarkable achievements.
Dr. Way is a pediatrician at Cincinnati Children’s Hospital studying the relationship between the immune system, pregnancy, and newborn health.
