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How the Immune System Works in Pregnancy
Your immune system protects your body from harmful invaders like bacteria and viruses while helping it heal from injuries. During pregnancy, it takes on an even more complex role. It allows your body to accept the foetus rather than rejecting it as a foreign threat. At the same time, it stays alert to fight infections and keep you protected. This balance involves two main components:- Innate (natural) immunity: This is your body’s rapid-response system, acting quickly to tackle threats within hours or days. It includes white blood cells like neutrophils and macrophages, as well as inflammatory chemicals called cytokines.
- Adaptive (acquired) immunity: The immune system develops specific antibodies to fight particular germs. It includes T cells and B cells, which retain memory of previous infections for a more effective response in the future.
Immune System Changes Throughout Pregnancy
Your immune system evolves as your pregnancy progresses, following what scientists call the “immunological clock of pregnancy.” These changes occur in distinct phases across the trimesters to support your baby’s development and maintain your health.First Trimester
In the early weeks of pregnancy, your immune system works hard to accept the embryo, which carries a mix of your genetic material and that of the father. Several adaptations occur to prevent your body from rejecting the foetus:- Immune cell increase: Your body produces more immune cells to support the pregnancy, such as natural killer (NK) cells and neutrophils. These cells help make sure the foetus isn’t attacked as a foreign entity.
- Movement of immune cells to the uterus: NK cells move to the uterus, surrounding the blastocyst (the early-stage embryo) to protect it and help it access nutrients from the endometrial tissue. This cell migration is important for successful implantation.
- Role of trophoblast cells: Trophoblasts are special cells that originate from a blastocyst and form a key part of the placenta. They release chemical signals called cytokines that attract immune cells like monocytes, neutrophils, and NK cells to the site where the embryo is implanting. These immune cells help build a healthy environment for the growing foetus.
- Pro-inflammatory environment: The first 12 weeks of pregnancy involve a mild inflammatory response in the uterus, similar to what happens when the body heals a wound. This temporary inflammation is important to help the embryo attach firmly to the uterine lining.
Second Trimester
Your immune system shifts to promote development while keeping infections at bay as your baby enters a phase of rapid growth:- Anti-inflammatory shift: The immune system becomes more anti-inflammatory to support foetal growth. Regulatory T cells (tregs) are critical here. They help protect the foetus, which carries genetic material from the father, from being mistakenly attacked by the mother’s immune system.
- Reduced activity of dendritic cells: Dendritic cells, which normally help activate immune responses, become less active in the uterus. This helps prevent your body from targeting the foetus.
- Maternal antibody transfer: Around the second half of pregnancy, your body starts passing IgG antibodies (a type of immune protection) to your baby through the placenta using a special protein called the neonatal Fc receptor. This transfer becomes stronger after 22 weeks and helps protect your baby against infections after birth.
Third Trimester
As you approach delivery, your immune system shifts again to prepare for labour:- Return to pro-inflammatory state: The immune system gradually reverts to a pro-inflammatory state, which is necessary to trigger labour. It helps the uterus contract, push out the baby and release the placenta. Labour may not occur naturally without this shift.
- Enhanced immune responses: Cells like neutrophils become more active to protect against infections. This makes sure that you and your baby stay safe as birth approaches.
The Placenta’s Role in Immune Protection
The placenta acts as an immune barrier, shielding your baby from infections while allowing important nutrients and antibodies to pass through:- Syncytiotrophoblast cells (specialised cells located in the outer layer of the placenta): These cells form a protective layer between maternal and foetal blood, resisting infections like herpes simplex virus (HSV) and cytomegalovirus (CMV).
- Toll-like receptors (TLRs): Found on placental cells, TLRs detect pathogens (organisms that cause disease) and trigger immune responses to fight infections.
- Epithelial cadherin receptors: These receptors in the cytotrophoblast layer (part of the placenta) help trap bacteria and prevent them from spreading to the baby.
- Complement proteins: The placenta produces proteins like C3 and C4, which improve its ability to defend against infections.
How Infections Affect Pregnancy
Despite these protective mechanisms, infections can pose significant risks during pregnancy. They may lead to complications like preterm birth or foetal anomalies. Both bacterial and viral infections can disrupt the delicate immune balance.Bacterial Infections
Bacterial infections can enter the maternal-foetal interface (where your tissues and your baby’s tissues interact) through the reproductive tract, maternal blood or other routes. This can trigger inflammation that may harm the pregnancy.- Inflammatory response: Certain bacteria can cause inflammation in the decidua (the uterine lining). These can induce preterm labour by triggering uterine contractions or cervical ripening.
- Preterm birth risk: Around 25 to 40 per cent of preterm deliveries are linked to bacterial infections. Inflammation from these infections can damage placental and foetal tissues.
- Placental microbiome: A recent research shows that a “placental microbiome” exists in healthy pregnancies, suggesting that some bacteria are normal and may support immune function. However, pathogenic bacteria can disrupt this balance.
Viral Infections
Viral infections can also threaten pregnancy, sometimes causing developmental issues without directly infecting the foetus:- Foetal inflammatory response syndrome (FIRS): Few viruses can trigger inflammation in the foetus. This leads to elevated levels of cytokines, which may affect the foetal brain and circulatory system. As a result, the risk of conditions like autism or schizophrenia later in life may increase.
- Increased susceptibility: Pregnant individuals are more prone to viruses like influenza and hepatitis E due to immune and physical changes.
Long-Term Effects of Infections
Infections during pregnancy can have lasting impacts on your baby’s health:- Neurological risks: High maternal inflammation from infections may increase the chances of neurological disorders in childhood.
- Developmental anomalies: Infections like the rubella virus or CMV in the first trimester can be particularly harmful. They may lead to miscarriage or cause congenital defects in the baby.
- Hearing loss.
- Hypoplasia (incomplete development of a tissue or organ).
- Spontaneous foetal loss.
- Small-for-gestational-age birth.
Development of the Foetal Immune System
Your baby begins developing its own immune system early in pregnancy:- Early immune development: Dendritic cells appear in the foetus as early as 9 to 13 weeks. Some scientists believe the immune system starts becoming active by the second trimester.
- Transfer of antibodies: Your IgG antibodies cross the placenta and provide your baby with early protection against germs. This transfer is important for building your baby’s immunity before and after birth.
- In-utero training: A foetus's immune system is primed or "trained" during pregnancy to recognise and respond to pathogens after birth.
Supporting Your Immune System During Pregnancy
You can take certain steps to support your immune system and reduce infection risks during pregnancy. These are some practical ways to do so:- Get vaccinated: The flu shot is recommended as pregnant individuals are more susceptible to severe infections like influenza or COVID-19.
- Take prenatal vitamins: Vitamins containing vitamin C, folic acid, and vitamin D support immune health. Vitamin C, found in citrus fruits and vegetables like red bell peppers, helps protect cells and fight infections.
- Avoid high-risk foods: Avoid foods like deli meats, sushi, raw eggs, soft cheeses, and mayonnaise, which can carry bacteria like Listeria.
- Manage stress: Chronic stress can weaken your immune system by reducing NK cell numbers. Try mindfulness, daily movement, or spending time with loved ones to ease stress.
- Prioritise sleep: Aim for 7 to 9 hours of sleep nightly to prevent immunodeficiency disorders. A consistent bedtime routine can help you rest better.
- Eat a balanced diet: Focus on fruits, vegetables, fibre, and omega-3 fats. Limit sugar and saturated fats to reduce inflammation and support immune function.
Why Immune Health Matters for You and Your Baby
Your immune system’s health directly impacts your baby’s developing immune system. A strong maternal immune system provides antibodies that help your baby build early immunity. On the other hand, excessive inflammation from infections or conditions like obesity can increase the risk of preterm birth or long-term health issues for your baby. So it’s important to look after yourself and follow a healthy lifestyle to support a safe and healthy pregnancy.Your body’s immune system during pregnancy protects you from infections while also supporting your growing baby. Always consult your healthcare professional for proper advice, especially if you have an autoimmune condition or concerns about infections. When your immune system is well-supported, your baby benefits too, both during pregnancy and after birth.
FAQs on The Pregnancy Immune System: How Your Body Balances Tolerance and Protection
- How does my immune system protect my baby?
Your immune system creates a safe environment by adjusting how it responds to threats. It also passes antibodies to your baby through the placenta to provide protection even after birth. - Can infections hurt your baby during pregnancy?
Yes, certain infections can cause problems like preterm delivery, miscarriage, or birth defects. Bacteria can trigger inflammation, and viruses like rubella can harm your baby’s development. - How can I keep my immune system strong during pregnancy?
Get vaccines, eat healthy foods like fruits and vegetables, take prenatal vitamins with vitamin C and D, and sleep 7 to 9 hours each night.
