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A foetus does not have a strong immune defence to protect itself from allergens and infections. For adults, the immune system is strong and can easily recognise and fight infections. But the foetus is mostly dependent on the mother's immune system for protection. This is when mothers' antibodies transfer to the foetus and protect the baby during pregnancy and after birth until the immune system matures.





In the first trimester, the foetal immune system is not functional enough to fight invaders; it prioritises only growth. In the second and third trimesters, T-cells and B-cells mature, but they still depend on the mother's antibodies. When antibodies are not transferred properly, it can lead to neonatal infections, sepsis, and long-term allergies or autoimmunity.





Foetal Immune Development

The foetal immune system grows and matures slowly. It primarily prioritises growth and tolerance over defence, so it relies on the mother's antibodies. If the foetus responds strongly in the early stage, it might attack maternal tissues and disrupt its growth. This is how the foetal immune system develops:

• Embryonic Stage: Around week 4, immune precursor cells develop in the yolk sac and shift to the foetal liver by week 7-8. These cells will later become red blood cells, white blood cells, and immune cells.
  
  
• First Trimester: By week 8-10, the thymus forms (a place for T-cell differentiation and maturation) and T-cell production begins, which helps control immune responses. Around week 11-12, macrophages appear in the gut. Immune cells are present, but they are not activated yet to prevent the foetus from attacking the mother or affecting its growth.
  
  
• Second Trimester: around week 13-20, B-cells and T-cells move into the spleen and bone marrow. Around week 19, Peyer's patches (immune tissue in the gut) begin to organise
  
  
• Third Trimester: The bone marrow becomes the main place of immune cell production. Neutrophils, infection-fighting cells, increase. Maternal IgG crosses the placenta from week 13 and peaks in the third trimester.
  
  

A baby's immune system remains immature at birth. Full maturity takes 2-3 years after birth. Premature babies are at a high risk of infection as they don't receive enough maternal antibodies.





Innate and Adaptive Immunity

• Innate Immunity: Innate immunity is the body’s first immune response. It includes skin, gut lining, phagocytes (neutrophils and macrophages) and proteins that destroy microbes. Macrophages are present from week 11, but they are limited in function and numbers. Neutrophils are very slow. Gut barriers form by week 10-28.
  
  
• Adaptive Immunity: These recognise intruders faster, remember them and respond quickly next time. It relies on T-cells and B-cells. T-cells become active by month 3-4, but they can't attack yet. B-cells produce small amounts of IgM, which is not effective for long-term protection.
  
  

Why the Foetus Depends on Maternal Antibodies

Foetuses rely on transplacental antibody transfer from their mothers from early on.

• Womb Environment: The most common reason is that the womb is a safe space for the foetus, so full immunity is not needed yet.
  
  
• Immature Immunity: The foetus cannot fight against pathogens or other allergen, so passive immunity acts as a protective antibody.
  
  
• Tolerance Balance: Foetal immune response could harm the mother or foetal growth, which can trigger inflammation or miscarriage.
  
  
• Safety: Maternal immunity is safe and efficient.
  
  

Antibodies and Types

Antibodies, also called immunoglobulin, are Y-shaped proteins produced by the immune system to fight against viruses, bacteria, or toxins. Immunoglobulins have five classes of antibodies.

• IgG: It is the most common and versatile antibody. It crosses the placenta to protect babies, fight toxins and viruses, and activate other immune cells.
  
  
• IgA: It is found in breastmilk, saliva, and tears. It protects mucosal surfaces like the gut, lungs, and saliva. It doesn't cross the placenta but supports the baby's barrier after birth.
  
  
• IgM: It's the first antibody to respond during infection. Its larger structure won't let it cross the placenta.
  
  
• IgE: It protects from allergy and parasites. It triggers histamine for itching and swelling. It also does not cross the placenta.
  
  
• IgD: It helps in B-cell maturation and signals early responses.
  
  

Transplacental Antibody Transfer

IgG is the primary maternal antibody that crosses the placenta to support the foetus. It's small and easy to transport via the neonatal Fc receptor. The transport increases in the third trimester. IgE also has four subclasses:

• IgG1: protects against viruses and bacteria
  
  
• IgG2: These fight against pneumonia
  
  
• IgG3: These antibodies fight against protein antigens and have strong inflammatory and antiviral responses
  
  
• IgG4: These block allergies and neutralise toxins
  
  

How Antibodies Cross the Placenta

Not every antibody can transfer to the placenta. IgG is the main transplacental antibody which can transfer due to its small size. The placenta is a disc-shaped organ attached to the uterine wall, connecting the mother’s blood to the baby’s blood through the umbilical cord.





Mother's blood flows through the intervillous spaces while foetal blood flows in the capillaries. These two are separated by a cell layer called the syncytiotrophoblast. It's the barrier between mother and foetus, responsible for exchanging nutrients, gases, hormones and antibodies.





The neonatal Fc receptor is a protein expressed on syncytiotrophoblast cells and foetal endothelial cells. FcRn recognises IgG antibodies and transports them across placental cells. Here's how it works.

• IgG Binding: Maternal IgG enters placental cells via endocytosis, which binds to FcRn.
  
  
• Transcytosis: The IgG-FcRn moves across the syncytiotrophoblast. These are safely transported without leakage or breakdown.
  
  
• Release: Once it reaches the foetal side, IgG is detached from FcRn and enters the foetal blood and starts its function.
  
  

The foetal immune system is immature to protect the mother and the baby from being attacked. But they still need antibodies to fight against and protect themselves from viruses and bacteria. Mothers' transplacental antibodies are transferred to protect against influenza, measles, rubella, tetanus, diphtheria, and other toxins. After birth, as the baby's immune system grows, the mother's antibodies start to decline.





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FAQs on How Antibodies Are Transferred To Foetal Immune System

How to increase a baby's immunity in the womb?

The baby's immune system is still under development inside the womb. They are mostly silent to avoid harming their mother and themselves. So to help the baby boost immunity to fight against pathogens and toxins, eat a healthy diet with nutrients and antioxidants. Add probiotic-rich foods to your diet. Take vaccinations at the right time.

Which fruits are rich in immunity?

Fruits, including oranges, lemons, grapefruit, kiwi, strawberries, blueberries, raspberries, blackberries, papaya, guava, pomegranate, mangoes, and watermelon, are rich in immunity.