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Would be helpful to have the exact reference for the assertion that ‘the adaptive immune system only really starts working 3 months or so after birth‘ if only to point out it’s utterly and wholly wrong. In fact, were it true, every human baby should die shortly after birth and our species should be long extinct. Why? Because each human baby ever born left its mother’s ~sterile womb to immediately deal with an existential necessity, how to incorporate microbes on its skin and various mucosal tissues and how to distinguish friend from foe among these lifelong partners. After all, living in a microbial world, we don’t have the choice to completely avoid contact with them. So, forget about vaccines, without a functioning adaptive immune system, a newborn would be simply unable to regulate this utterly unavoidable microbial colonization of its body. And happening ever since we first appeared on the scene, this process is so seamless and effortless, newborns so skillfully and quiescently incorporating microbes in and on their bodies, it entirely escapes our attention, and a rigorous, analytical examination of the process itself has until now largely escaped the notice of scientists. But it’s why vaccinating newborns with vaccines such as BCG vaccine , Hepatitis B vaccine , and (oral) Polio vaccine works.

Now, it’s a different matter entirely that some elements of the adaptive immune system, such as Marginal zone B-cell (1), aren’t fully developed or functional at birth. However, a newborn is replete with regular B cells (2, 3), which make up for some of this temporal deficiency. Newborns also have abundant, functional T cells (3). Data also show that newborns can make a variety of adaptive immune responses, even just as strong as those in adults (4, 5).

As well, Passive immunity transfers one aspect of maternal adaptive immune responses. Maternal antibodies that are transferred through the placenta in utero and through breast milk after birth are presumably those most useful and relevant to the newborn precisely because its chances of encountering the same instigating agents are the highest as well. Back in the 1950s and 60s, passive immunity was interpreted as a sign the newborn’s immune system is immature  because the concept of post-birth microbial colonization was simply not in the picture. Now it is, and it fundamentally alters the conceptual landscape. Rather than purely temporary defense against dangerous pathogens, maternal antibodies could instead help newborns sort friend from foe among their new microbial brethren.

Another compelling argument against immaturity or deficiency of newborn immune function is teleological. If a human newborn arrived into the world lacking the effective capacity to defend itself from microbial and other disease-bearing agents, evolution dictates that those other organisms would quickly exploit such a major loophole to only one end, finis of the human species as we know it. Our very history and existence clearly shows this didn’t happen.

Thus, newborn immunity shouldn’t be construed as immature or deficient but rather as different. Different because the newborn is involved in an existential physiological process of successfully incorporating microbes in and on its body, something older human bodies have already done. Understanding this concept is a must if we are to properly understand neonatal immunology, something we have thus far failed to do. Instead we made a category error by using children or adult adaptive immune responses as the yardstick to measure those of newborns. We can’t do so because microbe-replete children or adult bodies and not yet fully microbially colonized newborn bodies are different in kind, not degree.


1. Weill, Jean-Claude, Sandra Weller, and Claude-Agnès Reynaud. “Human marginal zone B cells.” Annual review of immunology 27 (2009): 267-285.

2. W. Marieke, et al. “Immunophenotyping of blood lymphocytes in childhood. Reference values for lymphocyte subpopulations.” The Journal of pediatrics 130.3 (1997): 388-393. http://repub.eur.nl/pub/54630/1-…

3. Comans-Bitter, Walker, J. C., et al. “Development of lymphocyte subpopulations in preterm infants.” Scandinavian journal of immunology 73.1 (2011): 53-58. https://www.researchgate.net/pro…

4. Marchant, Arnaud, et al. “Newborns develop a Th1-type immune response to Mycobacterium bovis bacillus Calmette-Guerin vaccination.” The Journal of Immunology 163.4 (1999): 2249-2255. Newborns Develop a Th1-Type Immune Response to Mycobacterium bovis Bacillus Calmette-Guérin Vaccination

5. Marchant, Arnaud, and Michel Goldman. “T cell‐mediated immune responses in human newborns: ready to learn?.” Clinical & Experimental Immunology 141.1 (2005): 10-18. https://www.researchgate.net/pro…