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We owe children the most protection when they are at their most vulnerable — early in their lives before they can protect and care for themselves. This vulnerability extends even to the invisible world of infectious diseases. From the moment they’re born, infants are thrust into a world of potentially dangerous microorganisms. While vaccines and natural immunity will quickly work to defend these babies, every second counts in the fight against contagion. Luckily, human beings possess the capacity to defend our newborn children from disease using an ingenious mechanism: maternal antibodies passed down from their birth parent. Maternal antibodies protect babies from the pathogens they might encounter soon after birth, allowing time to develop their own immunity. In this article, we’ll look at how babies benefit from maternal antibodies, how maternal antibodies work, where they come from, and how long they typically last.

The development of the human immune system occurs throughout fetal gestation, beginning as early as four weeks after conception. During development, both the innate and adaptive immune systems are established. The innate immune system is our first line of defense, and it is nonspecific, meaning responses are not selective to a particular pathogen. If something is considered foreign to the body, the innate immune system will respond. The adaptive immune system, on the other hand, is the “special ops” forces of the immune system. When the adaptive immune response is engaged, it is leveled at a specific pathogen. 

The womb is sterile, and although babies are equipped with many immune system protections at birth, it takes a few months after birth for these systems to fully mature. As such, shortly after birth, infant immune responses rely heavily on their innate immune system. But over time, after encountering pathogens in the environment and receiving vaccines, the baby’s adaptive immune system will be well prepared to step in and fight infections as needed. 

An important part of adaptive immunity is the antibody response. Antibodies are small proteins that target and bind to specific pathogens to ensure their destruction. While babies’ immune systems can make antibodies from birth, it takes time for the antibodies to be available in large numbers  against a wide array of pathogens. For these reasons, during the first months of life, babies rely on antibodies from elsewhere – this is where maternal antibodies come into play. 

A special gift: Maternal antibodies

During gestation, the fetus is protected from the external environment. The placenta limits what can reach the fetus, delivering necessary nutrients and hormones as well as protection in the form of maternal antibodies.

Maternal antibodies are antibodies generated by  the birth parent’s immune system and delivered to their baby to protect it from pathogens in the months after birth. Before birth, these antibodies are introduced through the placenta, and after birth, babies receive maternal antibodies in colostrum and breast milk. Immunity gained by introduction of antibodies generated by an external source is known as passive immunity. Passive immunity works well in protecting newborns, but it has some limitations. First, it is short-lived. After a few months passively transferred antibodies will go away and the infant will need to rely on self-generated immunity. Second, in the case of maternal antibodies, the type and quantity of antibodies passed to an infant depend on which pathogens or vaccines the child’s parent encountered prior to giving birth and how much antibody the parent made as a result of the encounter.

Maternal antibodies delivered through the placenta are primarily of a type known as “immunoglobulin G,” or IgG. IgG is the most common type of antibody found in blood. After being introduced via the placenta, the IgG will circulate in the fetal blood as well. The levels of IgG transported across the placenta increase throughout pregnancy, peaking in the third trimester. Maternal IgG remains in a baby’s blood for a few months after birth, protecting the baby from disease by binding to pathogens and marking them for destruction by the infant’s own white blood cells.

Following birth, the infant has an additional opportunity to receive maternal antibodies from colostrum and breast milk. These antibodies are primarily of the type known as “immunoglobulin A,” or IgA. Unlike IgG, IgA is most commonly found on mucosal surfaces, meaning the places where the inside of the body meets the outside of the body, such as the eyes, nose, mouth and intestines. IgA antibodies bind to microbes trying to make their way inside the body, causing them to get stuck in the mucous layer and preventing them from infecting the baby. 

Maternal antibodies and the immunization schedule 

While antibodies provided in breast milk will protect the baby until weaning, studies show that most antibodies received before birth will decrease over time and become undetectable by 6 to 12 months of age. Further, even if a baby is being breastfed beyond one year of age, the maternal antibodies in breast milk are not sufficient to protect a baby on their own. That is why it is important that during the first year of life as maternal antibodies wane, a baby’s own immune system continues to generate its own protective antibodies and immune cells. 

Medical experts, public health officials and scientists have planned the infant immunization schedule to account for both the presence and waning of maternal antibodies during the first year of life. For some vaccines, like those given at two, four and six months of age, the presence of maternal antibodies does not interfere with the immune response to the vaccine. As a baby receives these vaccines, their immune system produces antibodies and immune cells that take over the job of protection when maternal antibodies wane. For other vaccines, like live, weakened vaccines given as shots (e.g., MMR, chickenpox), the presence of maternal antibodies can interfere with the development of the immune response. As such, these vaccines are given at the end of the first year of life when maternal antibodies have significantly waned or completely disappeared. (For more information on this topic, see “Are maternal antibodies considered while making the vaccine schedule?” in the resources section of this article.) 

Leveraging maternal antibodies for even more protection

Historically, giving vaccines to pregnant people was a rarity. This was because we lacked scientific studies evaluating the safety of vaccines for this group and their unborn babies. However, over time, pregnant people have become more of a focus in clinical trials of new vaccines. This is important in part because pregnant people are at increased risk of experiencing severe disease if they are infected with some pathogens, like influenza and COVID-19. As such, being able to protect them through vaccination is important. Additionally, sometimes babies can suffer the consequences of infections experienced during pregnancy. Ebola is an example. (To find out more about clinical trials and pregnancy, see “Paving the way for maternal vaccination,” in the resources section.)  

These efforts have also been important because if a vaccine is safe to administer during pregnancy, we can leverage vaccination to ensure that babies receive maternal antibodies against the most dangerous pathogens for newborns. For example, the Tdap (tetanus, diphtheria, and pertussis) vaccine is recommended between 27 and 36 weeks of gestation because when new babies get pertussis, they can become severely ill or even die from this infection,  which is much more dangerous for the baby than anyone else in the family. A second benefit of getting vaccinated during pregnancy is that the pregnant person will be less likely to get a pertussis infection themselves, which is important not only so they are able to care for their baby, but also because when babies do catch pertussis, they often do so from a parent. The new respiratory syncytial virus (RSV) vaccine is another example of a vaccine recommended during pregnancy to protect the newborn. However, in the case of RSV, a monoclonal antibody product is also available to protect babies right after birth. 

With this said, some vaccines should still be avoided during pregnancy, such as those that contain live, weakened viruses, like MMR and chickenpox vaccines. While these vaccines have not been shown to harm pregnant people or their developing babies, the recommendation is to avoid them due to theoretical risks associated with the viral replication that takes place in order to generate immunity.

In sum

Maternal antibodies play a vital role in providing passive immunity to babies during the first months of life when they are at their most vulnerable. This evolutionary method of protecting babies from the dangers of communicable diseases has kept humans alive for eons. Using vaccination as a tool for enhancing maternal antibody production allows us to safeguard even more lives. And, by heeding the recommendations for vaccinating themselves during pregnancy, a parent has the opportunity not only to protect themselves from potentially severe infections when they are vulnerable, but also to protect their baby during a period of vulnerability right after birth. Their vaccine can deliver the precious gift of antibodies.
 

Related resources 

Development of the Immune System (webpage)  

Vaccines during pregnancy: The history and reasons behind the recommendations (article)

Are maternal antibodies considered while making the vaccine schedule? (video)

Paving the way for maternal vaccination (video) 
 
How Do Antibodies Work? (animation)   

Types of Immunity (webpage)  

Recommended Vaccinations for Infants and Children, Birth – 6 months (webpage)