Most of us have heard that breastfeeding supports a baby’s immune system. But what does that really mean? And how does it work?

A baby is born with an immature immune system that continues to develop and mature through the first five years of life. This makes young babies and children potentially more vulnerable to viruses and other illnesses. A breastfeeding mother is able to support her children through these first exposures to illness by lending the support of her healthy, adult immune system.

Your breastmilk is jam-packed with antibodies and other bioactive cells. In fact, one study found there to be over 3,000,000 such cells in just one teaspoon of breastmilk, so there’s great value in any amount of milk your baby receives!

These immune factors aren’t static; they are a dynamic process. We don’t know for sure how this happens, but the hypothesis is that a baby signals their immune needs to their mum while they feed. During a breastfeed, the vacuum created by the baby while they are sucking causes some of the baby’s saliva to enter mum’s body in a ‘backwash’ type situation. If the baby has been exposed to an immune challenge like a virus, this saliva will trigger an immune response from mum’s mature immune system, which in turn will make immune cells (leukocytes) that will be passed back to the baby via the next portion of breastmilk. We know that leukocyte levels are found in higher concentrations in breastmilk when the baby is unwell, supporting their recovery through borrowed access to mum’s mature immune system.

What does that look like in reality?

This doesn’t mean that your baby will never catch a cold or that milk is a magic elixir that repels all illness. What it does mean is that, when faced with a challenge, a breastfed baby has an enhanced ability to fight it off and recover more quickly. For example, in the UK, over half the hospital admissions for diarrhoea and over a quarter of the admissions for lower respiratory tract infections could be prevented through exclusive breastfeeding.

The immune system isn’t just there to fight off acute viral infections; your breastmilk can also support lifelong health in your baby. Breastfeeding contributes to lower levels of obesity throughout childhood (and probably beyond). And we are just starting to understand how breastfeeding contributes to the infant microbiome, which has implications for gut health and immunity.

All of this data is on a ‘population level’ and describes the expected and average benefit for breastfed babies. It’s important to understand that this doesn’t mean that every individual breastfed baby will be healthier than every individual formula-fed baby.

Major Bioactive Factors in Human Milk

While milk is an ever-changing and evolving fluid, these are the major immune supporting factors found in breastmilk, and what they do for your baby’s health.

(Source: Ballard O, Morrow AL. Human milk composition: nutrients and bioactive factors. Pediatr Clin North Am. 2013 Feb;60(1):49-74. doi: 10.1016/j.pcl.2012.10.002. PMID: 23178060; PMCID: PMC3586783.)

Component	Function
Cells
Macrophages	Protection against infection, T-cell activation
Stem cells	Regeneration and repair
Immunoglobulins
IgA/sIgA	Pathogen binding inhibition
IgG	Anti-microbial, activation of phagocytosis (IgG1, IgG2, IgG3); anti-inflammatory, response to allergens (IgG4)
IgM	Agglutination, complement activation
Cytokines
IL-6	Stimulation of the acute phase response, B cell activation, pro-inflammatory
IL-7	Increased thymic size and output
IL-8	Recruitment of neutrophils, pro-inflammatory
IL-10	Repressing Th1-type inflammation, induction of antibody production, facilitation of tolerance
IFNγ	Pro-inflammatory, stimulates Th1 response
TGFβ	Anti-inflammatory, stimulation of T cell phenotype switch
TNFα	Stimulates inflammatory immune activation
Chemokines
G-CSF	Trophic factor in intestines
MIF	Macrophage Migratory Inhibitory Factor: Prevents macrophage movement, increases anti-pathogen activity of macrophages
Cytokine Inhibitors
TNFRI and II	Inhibition of TNFα, anti-inflammatory
Growth Factors
EGF	Stimulation of cell proliferation and maturation
HB-EGF	Protective against damage from hypoxia and ischemia
VEGF	Promotion of angiogenesis and tissue repair
NGF	Promotion of neuron growth and maturation
IGF	Stimulation of growth and development, increased RBCs and haemoglobin
Erythropoietin	Erythropoiesis, intestinal development
Hormones
Calcitonin	Development of enteric neurons
Somatostatin	Regulation of gastric epithelial growth
Anti-microbial
Lactoferrin	Acute phase protein, chelates iron, anti-bacterial, anti-oxidant
Lactadherin/ MFG E8	Anti-viral, prevents inflammation by enhancing phagocytosis of apoptotic cells
Metabolic hormones
Adiponectin	Reduction of infant BMI and weight, anti-inflammatory
Leptin	Regulation of energy conversion and infant BMI, appetite regulation
Ghrelin	Regulation of energy conversion and infant BMI
Oligosaccharides & glycans
HMOS	Prebiotic, stimulating beneficial colonisation and reducing colonisation with pathogens; reduced inflammation
Gangliosides	Brain development; anti-infectious
Glycosaminoglycans	Anti-infectious
Mucins
MUC1	Block infection by viruses and bacteria
MUC4	Block infection by viruses and bacteria

How does breastfeeding support my baby's immunity?

For our Baby Feeding Journey, Unii Expert Wendy Lever explains how breastfeeding supports your child’s immunity.