Biomedical Research

Neonatal Immunology: Our Immune Systems in the Weeks After Birth

The immune systems of neonates are, unsurprisingly, different and less developed than those of adults.

By Michelle Li

Published 12:08 AM EST, Sun July 4, 2021


Neonates are newborn infants that are four weeks old or younger. These first four weeks of an infant’s life are when the infant is at highest risk of dying. At this stage in life, neonates do not have fully developed immune systems and are more susceptible to different infections. Of the 5 million infant deaths that occur each year, 1.5 million are due to infections, making it important to understand the developing immune system of neonates (Tregoning). 

Part of understanding the immune systems of neonates is first understanding the transition form the sterile womb to an unsterile environment during birth. The fetal immune system is suppressed in the womb in order to limit interference with the mother’s immune system. While this provides stability before birth, the arrangement changes the second after birth, when the newborn enters the unsterile environment of the world. In addition to the risks of being exposed  to bacteria, the fetal immune system after birth (which was previously suppressed) is antigenically inexperienced; it does not yet have experience responding to different pathogens, which increases the infants susceptibility to infections (“Development of the Immune System”). Therefore, after birth, neonates depend on “passive immunity” for protection, as their own immune systems develop.

Passive Immunity

Neonates depend on antibodies from the mother for protection from different antigens. This is called “passive immunity,” as antibodies from the mother are passed down to the baby passively through the placenta, rather than the antibodies being created by the infant themselves. Most of the antibodies produced by the mother’s immune system cross the placenta during the third trimester, which ensures that there are high levels of antibodies after birth. This also explains the low levels of antibodies in premature babies; the timing of the birth does not allow for the same amount of antibodies to be transferred, making premature newborns more vulnerable to infections compared to full-term newborns. Additionally, breastfeeding is another form of passive immunity that allows for the passing of antibodies to infants (“Development of the Immune System”). 

Passive immunity only provides short term protection for neonates. The antibodies transferred through the placenta or breast milk are generally immunoglobulin A or G (IgA or IgG). Some of these maternal antibodies protect against measles, mumps, rubella, etc (“Immunity: Active, Passive, and Delayed”). The antibodies transferred passively from the mother to the child either through the placenta or breast milk only protection for the first few months of the infant’s life. This allows the infant’s immune system to develop and start working while keeping the infant protected (“Development of the Immune System”).

The Immune System At This Time

Newborns have a limited quantity of phagocytic cells (types of white blood cells such as neutrophils and macrophages), which are important for innate immunity (the nonspecific immune response immediately after the appearance of an antigen). During an infection, the immune system’s response will be limited by the quantity of neutrophils and macrophages. As a result, the pathogen will commonly overtake the immune system, and the infant will require medical care (“Development of the Immune System”).

In addition, there is also adaptive immunity, which is the specific immune response that occurs after the innate immunity system fails; it is the system that protects the body by remembering and destroying pathogens. As the newborn’s immune system is inexperienced, every pathogen is new, resulting in the immune response taking a longer time to develop. The fact that every pathogen is new also means that there are no memory immune responses, which affects antibody production (“Development of the Immune System”). The process of producing antibodies is less efficient in newborns compared to adults. Some B cell (a type of white blood cell) responses require T cells to produce antibodies. The interactions between T cells, which attack specific antigens, and antigen-presenting cells, which present antigens for recognition, are less effective and stimulating in newborns. There are lower levels of cytokines (which regulate the immune response) produced by T cells. Furthermore, the levels of types of T cells are different in newborns than in adults. For instance, there are lower levels of cytotoxic T cells, which are responsible for killing virus infected cells. These factors influence the levels of antibody production. For B cell responses that don’t involve T cells, B cells recognize the repeating proteins on the surface of a pathogen; this response is also reduced in newborns, resulting in increased susceptibility to bacteria (“Development of the Immune System’).


The reduced immune response of newborns affects the efficacy of vaccines, as there is reduced recognition of vaccine antigens as foreign. Therefore, there are also fewer protective memory responses induced by vaccines, making vaccines themselves less effective in newborns compared to adults with developed immune systems (Tregoning). However, this does not mean that early vaccinations are ineffective. They still aid in protecting against diseases, and they become more effective over time as the newborn’s immune system develops (“Development of the Immune System”). 

In fact, as the protection from passive immunity fades over a number of months, vaccinations are required to maintain protection against different antigens. The fading of maternal antibodies is also why there are certain required vaccinations after set periods of times; for instance, the MMR vaccine is required after 1 year of life (“Immunity: Active, Passive, and Delayed”).


The immune systems of neonates are, unsurprisingly, different and less developed than those of adults. As a result, newborns depend on passive immunity (antibodies passed down through the placenta or breast feeding) for protection against infections. The processes in the immune system itself are also different in newborns, which affects the immune system’s capabilities. The increased susceptibility to infections in newborns makes it all the more important to understand the neonatal immune system.

Michelle Li, Youth Medical Journal 2021


“Development of the Immune System.” Children’s Hospital of Philadelphia, Accessed 31 May 2021.

“Immunity: Active, Passive, and Delayed.” World of Microbiology and Immunology, edited by Brenda Wilmoth Lerner and K. Lee Lerner, Gale, 2007. Gale in Context: Science, Accessed 31 May 2021.

Tregoning, John. “Neonatal Immunology.” British Society for Immunology, Accessed 31 May 2021.


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