Vaccines and Immunity that Pass from Mother to Baby

How does immunity from the mother get passed to the fetus?

The membranes of the placenta create a barrier that allows some substances to cross to the fetus while keeping other substances out. For example, the placenta generally does not allow bacteria to enter the fetal bloodstream,¹ and can even provide a barrier against some viruses.² In contrast, the placenta allows water, gases, and nutrients to pass to the fetus. It even transfers some large molecules, like maternal antibodies, to the fetus. Antibodies are enormous compared to the molecules and chemicals that commonly cross the placenta. Therefore, they need a specific receptor, called the neonatal Fc receptor (FcRn), to actively move them into the umbilical cord and fetal blood stream.³ While there are five different classes of antibodies, the FcRn specifically transfers IgG, the main circulating antibodies built up over the mother’s lifetime.⁴ IgG antibodies circulate in the fetus’s blood and provide protection against specific germs and toxins.

This CT scan of a pregnant mother shows the placenta slightly above the fetus near the mother's spine. The placenta is the main connection between the mother and the fetus. The placenta guards the baby from most outside infections, and even has receptors to transfer antibodies from the mother to the unborn baby. (Image Credit: Mikael Häggström, 26 July 2017)

17

Weeks

As the baby grows, the mom’s body quietly passes immune protection— maternal antibodies steadily build in the fetal bloodstream from about 17 weeks on, with a sharp surge in the month before birth.⁵ ⁶

Can a mother influence how much protection she gives to her baby?

The more antibodies against a particular germ that the mother has circulating in her own bloodstream, the more antibodies that will transfer to her baby. That is why the Center for Disesase Control recommends getting a vaccine against tetanus, diphtheria, and pertussis (Tdap) between 27 and 36 weeks’ gestation every time a mother is pregnant, whether or not she already has had the vaccine.⁷ The vaccine stimulates the mother’s immune system to create more antibodies; therefore, more antibodies transfer to the fetus, helping protect him from whooping cough, caused by the pertussis virus, after birth.  Similarly, when mothers get the flu vaccine late in pregnancy, it substantially reduces the risks of infection and hospitalization in newborns within the first 6 months after birth!⁸ ⁹

A maternal vaccination against influenza has been shown to substantially reduce risks of infection and hospitalization in newborns for their vulnerable first six months outside the womb.¹⁰ ¹¹

Since mothers can pass antibodies to their children while they are still pregnant, researchers continue to develop vaccines against harmful diseases that can be deadly to newborns. These efforts led to the recent approval of a maternal vaccine against respiratory syncytial virus (RSV), approved for use after 32 weeks’ gestation.¹² Currently, vaccine candidates against Group B strep are in clinical trials, with encouraging results.¹³ ¹⁴

Why do some antibodies cross the placenta while others don't?

Researchers are still investigating the reasons why some antibodies transfer to the fetus efficiently while others do not, although many factors appear to influence this shared protection. The mother’s distribution of antibodies certainly plays a role.¹⁵ But recent studies have also investigated what makes for the most optimal transfer. One study found the placenta preferentially transfers antibodies that stimulate the fetus’s own immune system to activate natural killer cells.¹⁶ Therefore, the placenta is designed to selectively transfer maternal antibodies that will best train the fetus’s immune system to fight future infections itself. There are now ongoing studies to design antibody therapies which can best transfer from mother to fetus.¹⁷

The respiratory syncytial virus (RSV), a common contagious virus that infects the human respiratory tract. (Image Credit: NIAID)

How do antibodies in the breastmilk protect the baby?

Even after birth, the infant continues to get maternal antibodies through the breastmilk. The most common antibody in breastmilk is IgA, which protects against germs in the respiratory system and gut. The IgA molecules in breastmilk are part of a complex, called secretory IgA which protects them from breaking down in the newborn’s gut.¹⁸ ¹⁹ These IgA molecules attach to the lining of the intestine and protect the infant against ingested bacteria. While the newborn is still developing his capacity to produce IgA, maternal antibodies fill the gap, protecting against microbes like E. coli, Shigella and Salmonella.²⁰ ²¹ But beyond antibodies, breastmilk also contains a number of other components which help strengthen a newborn’s immune responses. These include the mother’s own immune cells and substances which bind pathogens before they can ever infect the baby!²²

T cells are white blood cells important for immune memory, helping the body respond more quickly if it encounters the same infection again. (Image Credit: NIAID)

What role do the mother’s cells play in the baby’s developing immune system?

The cells transferred from the mother’s breastmilk are predominantly immune cells which bring the power of the mother’s mature immune system to help protect the newborn.²³ ²⁴ These cells include innate immune cells, like neutrophils and macrophages, which compensate for the inability of the newborn to mount a strong response against germs.²⁵ ²⁶ They also include T cells specific to the gut, which travel from the mother’s gut specifically to be delivered through breastmilk and contribute to the protection of the newborn’s vulnerable digestive tract.²⁷ ²⁸ Memory T cells, established during the mother’s lifetime of built immunity, lend that memory to the child as well.²⁹

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Vaccines and Immunity that Pass from Mother to Baby