Baby’s Immune System on Breast Milk

It is known through countless studies that breastfed babies have a lower instance of infection. There is also a huge increase of infection in weaning children at the time when the protection of breast milk is diminished. So, not only is breast milk an amazing source of nutrition for newborns and infants, it provides the baby’s immune system with some great protection.

So, how does this work?


There’s lots of talk about antibodies being passed from mother to child through breast milk. Well, that’s no myth.

Before the baby is born, the baby has some immunologic defenses but they are very underdeveloped. In preparation for the baby’s entry into the world, the mother’s immunoglobulin (antibody) type G, called IgG, moves across the placenta and into the baby’s body to offer some protection in its first stages of life.6 This type of antibody is the smallest of all the antibody types (and why it can pass through the placenta), but it packs a huge punch. IgG antibodies are capable of carrying out ALL of the functions that antibodies are able to do, and they also play a fundamental role in fighting bacterial and viral infections. Therefore, this transfer of antibodies from mother to baby before birth gives the baby a huge curtain of protection from lots of diseases before entry into the world.

I’m going to throw this link in right now because this is an important example IgG transfer from mother to baby offering HUGE protection to baby: Why pregnant mothers should get a pertussis vaccine to protect baby from whooping cough as an infant.

After the baby is born, these IgG maternal antibodies decline within the first 6-12 months of life, however, the mother can still offer the baby some antibody protection via the breast milk. When babies are first born, they are deficient in the main antibody that protects mucosal membranes.1 This antibody is called secretory immunoglobulin A (sIgA). And human breast milk contains huge quantities of sIgA.

This antibody can offer the baby BIG protection from the outside world. Why? IgA protects mucosal surfaces–surfaces of the body that are exposed to the outside world and are particularly vulnerable to infection. These surfaces include thin and very permeable barriers such as the lungs, gut, eyes, nose, throat, mouth, and reproductive organs. It’s not surprising that the vast majority of infectious agents invade the body through these surfaces, so protection from this mucosal antibody type is hugely beneficial.

The mother’s previous exposure to some infectious pathogens, including some she’s been vaccinated against and formed antibodies to, can pass through the milk and bind those pathogens potentially entering the baby’s body to stop them from infecting cells. This type of antibody works by taking up and destroying pathogens, and can also help in limiting the damaging effects of tissue inflammation that occur when other antibody types aide in eliminating the infection.

Studies show that human breast milk can block the attachment of otitis media-causing strains of pneumococci and H. influenzae to cells in the throat.3 Such studies suggest that the milk IgA antibodies may have anti-attachment capacity, meaning certain bacteria are not able to attach to the mucosal membranes and cause infection.3

Several studies show that mothers can protect babies from an invasion of Enterobacteriacae, such as E. coli,  V. cholerae, and Shigella.4,5 All of these bacteria invade the gut and can cause severe disease in babies. There is also strong protection from these bacteria entering the urinary tract and causing urinary tract infections in breastfed babies.5

Another study of importance suggests that there are numerous immunosuppressive factors found in colostrum, the first milk that comes in after the birth of the baby.2 The immunosuppressive factors help protect the newborn’s immune system from overstimulation by a large number of environmental antigens (foreign proteins that the body recognizes as attackers), such as allergens.2 The colostrum also provides the baby with both antibodies and immune cells against diseases that the mother has been vaccinated with.2


Measurable levels of leukocytes (white blood cells) have been found in breast milk, especially in colostrum.6 Macrophages and neutrophils are the most common leukocytes found in the milk.6 Both of these cell types have important immune system functions. Neutrophils ingest microorganisms, break them down, and eliminate them. Macrophages also play an important role in digesting microorganisms but also help to pass the information along to other immune system cells so that more cells are aware of intruders.

Lymphocytes make up the other 10% of the white blood cells in breast milk.6 These cells include T cells, natural killer cells, and antibody-producing B cells. Lymphocytes from the mother have been found in the draining lymph nodes of the infant.There is evidence (based on animal studies) to suggest that these cells pass into the digestive tract of the baby, get absorbed by the baby’s gut mucosal lining, and can influence the baby’s immune system.1 The amount of all these cell types from the mother in the infant’s body, however, decreases by ten-fold in mature breast milk.6

One study found an association between the number of CD4+ and CD8+ cells and breastfeeding.11 The “CD4+” and “CD8+” are T cell receptors, and can be found on many immune system cells. Basically, they are receptors for the T cells to dock on to. You’d see a higher number of these receptors when you have a higher number of immune system cells. Babies exclusively breastfed had higher numbers of these cells at 8-10 months of age, and the group concluded that breastfeeding may correlate to a current and long-term immune-modulating effect on the developing cellular immune system.11

Antimicrobial Factors

There are some factors in breast milk that have antimicrobial effects. An enzyme called lysozyme is passed through breast milk. This enzyme can help kill off some types of bacteria. Also, one of the most abundant proteins in breast milk is called lactoferrin, a protein that can help limit bacterial growth.

Increase in response to childhood vaccination

There have been several studies conducted that compare the immune responses to vaccines between breastfed and formula-fed babies. It seems that the results of these studies are all over the board.

One study followed 10,000 infants vaccinated with the conjugate vaccine: Haemophilus influenzae type b and tetanus toxoid. They found a significant increase in antibodies made to the diseases in the vaccine in infants who were breastfed for at least six months.8 However, there are several studies showing there is no effect on immune system response to vaccines by breast milk.

There have been some studies that even show breast milk has a negative effect on the immune response to vaccines. There was a meta-analysis that included three studies, which suggests that the response to the live viral vaccines may actually be inhibited by the secretory IgA antibodies in breast milk.9 The meta-analysis followed 500 babes’ responses to the live attenuated oral rotavirus vaccine. The immune response to the vaccine depends on the virus being able to replicate, and this group postulates that this may be inhibited by the secretory IgA antibodies in the breast milk. This means that the IgA antibodies from the mother are so good at keeping the disease in check, that the baby’s body doesn’t make a response to the virus in the vaccine because it never really has to. However, when the dose of the vaccine is increased, the baby’s body is able to make the proper response to the vaccine.

Influence in immune system development

There is evidence to suggest that breastfed babies have more mature immune systems than formula-fed babies. Researchers have been able to measure larger thymus glands in babies who were exclusively breastfed. The thymus is a central organ in the immune system. One major role of the thymus is the development of T cells. It takes immature T cells and removes those unfit for the immune system, then releases functional mature T cells. And although the clinical significance of the size of the thymus is unknown, because it plays such a huge role in the immune system, the larger size may suggest a more mature immune system.6

The decrease in autoimmune disorders and allergies

Breastfed babies may have lower instances of autoimmune disorders, including atopic (genetically predisposed) allergies.6 This, however, is very hard to prove and study.

A recent meta-analysis (using studies from 1966-2000) explored the relationship between breastfed babies and allergic rhinitis.6 Using many studies, the researchers concluded that babies breastfed exclusively for the first three months of life were protected from allergic rhinitis.6 However, the association was substantial but not statistically significant.

Several groups looked at breastfeeding and the risk of Crohn’s disease. Studies suggest that formula feeding was independently associated with increased risk of Crohn’s disease but not ulcerative colitis.6

Studies also show that countries with low rates of breastfeeding past the age of 3 months had the highest rates of Insulin-Dependent Diabetes Mellitus.6

But, I want to let it be known that the evidence in the above autoimmune and allergy studies is far from conclusive.

Breast milk has some truly amazing properties. But I’m not here to lecture you on the importance of breastfeeding. Just do what’s best for you and your family. Babies are happy just as long as they have full tummies!


  1. Hanson, L.A. and T. Söderström. “Human Milk: Defense Against Infection”. Prog Clin Biol Res. 1981. Vol. 61. pp. 147-59.
  2. Chernishov, V.P. and I.I. Slukin. “Mucosal Immunity of the Mammary Gland and Immunology of Mother/Newborn Interrelation”, Arch Immunol Ther Exp (Warsz.). 1990. Vol. 38(1-2). pp. 145-64.
  3. Hanson, L.A, et. al. “The Immune Response of the Mammary Gland and Its Significance for the Neonate”. Ann Allergy. December 1984. Vol 53(6 pt 2). pp. 576-82.
  4. Hanson, L.A, et. al. “New Knowledge in Human Milk Immunoglobulin”. Acta Paediatr Scand. September 1978. Vol. 67(5). pp. 577-82.
  5. Hanson, L.A, et. al. “Secretory IgA Antibodies to Enterobacterial Virulence Antigens: Their Induction and Possible Relevance.”. Adv Exp Med Biol. 1978. Vol. 107 pp. 165-76.
  6. Jackson, K.M. and A.M. Nazar. “Breastfeeding, the Immune Response, and Long-term Health.” Journal of the American Osteopathic Association. 1 April 2006. Vol. 106(4). pp. 203-7.
  7. Newman, Jack, M.D. “Breastfeeding and Illness.” International Breastfeeding Center. 2014.
  8. Greenberg, D.P., et. al. “Immunogenicity of Haemophilus influenzae type b tetanus toxoid conjugate vaccine in young infants”. The Kaiser-UCLA Vaccine Study Group. J Infect Dis. 1994. Vol. 170. pp. 76-81.
  9. Pichichero, M.E. “Effect of breast-feeding on oral rhesus rotavirus vaccine seroconversion: a meta-analysis”. J Infect Dis. 1990. Vol. 162. pp. 753-755.
  10. Hasselbalch, H., et. al. “Decreased thymus size in formula-fed infants compared with breastfed infants”. Acta Paediatr. 1996. Vol. 85. pp. 1029-1032.
  11. Jeppesen, D.L., et. al. “T-lymphocyte subsets, thymic size, and breastfeeding in infancy.” Pediatr Allergy Immunol. April 2004. Vol. 15(2). pp. 127-32.
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