Infants are born with immature immune systems that develop over time through a combination of genetic factors, environmental exposure, and nutrition. Breast milk contains a variety of components associated with immune function that are not found in formula. Research suggests these may support aspects of immune development in infancy, though outcomes vary between individuals and settings.
Immune-related components in breast milk
The following components found in breast milk have been studied in relation to immune function. Their clinical significance varies and continues to be investigated.
Secretory IgA (sIgA)
Antibody
The dominant antibody in breast milk. Research suggests it lines the gut and respiratory mucosa and may bind to certain pathogens. Levels are highest in colostrum (first milk) and appear to reflect pathogens the mother has recently encountered.
Human Milk Oligosaccharides (HMOs)
Complex sugars — many distinct structures
HMOs act as prebiotics, selectively supporting beneficial gut bacteria. They may also interfere with pathogen binding in the gut. Some HMOs are now synthesised and added to certain formula products.
Lactoferrin
Iron-binding glycoprotein
Binds iron, which certain bacteria require for replication. Research also suggests it may have antibacterial and anti-inflammatory properties. It is present in breast milk and not found in standard formula.
Lysozyme
Enzyme
May disrupt the cell walls of certain bacteria. Found at much higher concentrations in human milk than in cow's milk-based formula.
Living white blood cells (leukocytes)
Cells (macrophages, neutrophils, T-cells)
Breast milk, particularly colostrum, contains white blood cells. Their functional role after ingestion is still being studied. Formula, as a sterile product, does not contain living cells.
Cytokines and growth factors
Signalling molecules
Various signalling molecules present in breast milk may play a role in immune calibration during early development. Research in this area is ongoing.
What the research shows
Epidemiological studies have found associations between breastfeeding and lower rates of certain infections in infancy, including some respiratory and gastrointestinal infections. These are observational associations from population-level studies, not direct causal evidence. Outcomes vary widely between individuals, and many factors influence infant health including genetics, environment, healthcare access, and socioeconomic circumstances.
How to read this evidence
Population-level statistics describe average patterns across large groups. Individual outcomes depend on many factors. A formula-fed baby in a low-pathogen environment with attentive healthcare may have fewer infections than a breastfed baby in a high-exposure setting. These statistics do not predict any individual baby's health.
If you are formula feeding
Formula does not contain the same biological components as breast milk. That is an honest description of a difference — not a reason for concern. Formula-fed babies develop healthy immune systems. Their immune development occurs through the same mechanisms available to all infants: environmental exposure, genetics, and time.
- →Vaccines are the most evidence-supported immune investment available, regardless of feeding method
- →Some formulas now include synthesised HMOs — early research into their effects is ongoing
- →Probiotics show modest evidence for reducing infection frequency in some formula-fed infants — discuss with your pediatrician
- →If you want to continue or increase breastfeeding, a lactation consultant can help assess what is realistic
Sources: WHO, American Academy of Pediatrics, AHRQ Breastfeeding and Maternal and Infant Health Outcomes systematic review.