How Does the Baby’s Immune System Develop?

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5 min read

Apr 16, 2025

As your newborn enters the world, they’re immediately exposed to countless invisible germs—bacteria, viruses, and other pathogens. But how does such a tiny body defend itself, and where does their immunity actually come from?

Well, it all lies in a strong immune system of your baby that has multiple stages of development…prenatal stage, after-birth stage, and postnatal stage.

But, what are these stages and how do they help in the development of your baby’s immune system? Let’s learn about it in detail below.

Prenatal Immune Development

For those who don’t know, your baby’s immune system starts developing in the very early stage. In fact, it’s formation starts forming just a very few weeks after conception. By week 4, special cells start developing that later convert into important organs such as bone marrow, spleen, and thymus.

By week 12, small White Blood Cells start to develop. These cells help a baby fight off infections against dangerous pathogens. However, your baby’s immune system is still not fully developed before birth. Here’s a timeline of what actually happens before birth:

Around week 20, special immune molecules called antibodies start moving from the mother to the baby through the placenta.
By week 32, the baby is getting even more Immunoglobulin G (IgG).
By the time a baby is born, they already have 60–70% of their mother’s antibodies.

These antibodies help protect the baby for the first few months after birth. That’s also why babies born earlier (before 34 weeks) have weaker immune systems.

Immune System at Birth

At this point, your baby has some protection from their mother, but the immune system is still not developed completely. Then, the first line of defence develops at birth. This includes skin & mucus, white blood cells, and inflammation.

But, the newborn’s system has no strong defence against new proteins or viruses. This is why they get sick easily and need extra care in the first few months after birth. However, the good news is they can get extra antibodies through breast milk.

Postnatal Immune Development

Now comes the final stage of immune system development. It is the time when your baby’s immune system fully develops. This stage is called the postnatal immune development. In it, baby makes the immune system stronger through breastfeeding, vaccines, and gut bacteria.

Breast milk acts as a natural immune booster that contains the antibodies IgA, IgG, IgM. All of these antibodies has the ability to protect against virus and bacteria. Then, there are WBCs and prebiotics in the breast milk.

Apart from breastfeeding, vaccines can also strengthen your baby’s immune system. Vaccines like Hepatitis B, DTaP (diphtheria, tetanus, pertussis), Polio, Hib, Rotavirus, Pneumococcal, and Meningococcal are a great defense against diseases like pneumonia, measles, and whooping cough.

Lastly, babies fully develop their immune system from the gut bacteria. This is because 70% of the immune system is in the gut.

Final Thoughts

By the time your baby turns one year older, their immune system is much stronger. But it still needs a few years to mature completely, and that’s why doctors recommend parents to provide their babies with good nutrition, vaccines, and a healthy environment, especially in the first few months after birth.

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