The immune system of the lactating breast is made up of nested complex adaptive systems

Lactocytes take up plasma components and manufacture constituents of breast milk to secrete a nutritive and immune-factor-rich fluid into the alveoli and duct lumens. The mammary gland immune system provides defense against both endogenous tissue damage and exogenous infection, in the breast, the milk, and the infant. Mechanical pressures affect or interact with each element of the mammary immune system.

Applying a complexity lens, clinical inflammation emerges as a host immune response to physiological stress, which then acts to downregulate perturbation and restore homeostasis in the lactating breast.

The perturbations or disruptions within the mammary gland immune system which lead to clinical inflammation result from a complex network of interactions, including between

1. Maternal immune system (e.g. regulated by inflammatory cells and factors from bloodstream, also by lymphatic vasculature)

2. Breast stroma immune system (including immune cells and breast stroma microbiome)

3. Human milk. The milk itself is made up of multiple complex adaptive systems, which are all parts of the immune system and its responses, including

   • The microbiome. You can find out about this here.

   • Somatic cells in the milk (white cells and exfoliated mammary epithelial cells from the duct lining). You can find out about these here.

   • Oligosaccharides

   • Exosomes

   • Metabolome (shaped in part by the microbiome).

Clinical strategies for supporting and stabilising the complex adaptive systems of the mother's immune response need to be multi-lateral

Strategies for both prevention and treatment of breast inflammation in a lactating breast are

• Multi-lateral, and

• Promote resilience (or stabilise systems) by

  • Downregulation of emergent feedback loops

  • Upregulation of other protective feedback loops.

Rapid stabilisation is much more likely if disruptive external factors which promote inflammation are removed, so that mammary gland resilience is optimised.

You can find the five key elements to stabilising the maternal mammary immune system starting here.

You can find out why the breastfeeding mother-baby pair is a complex adaptive system here.

Recommended resources

The microbiome of the lactating nipple-areolar complex is part of the skin's immunoregulatory system

Selected references

Douglas P. Re-thinking benign inflammation of the lactating breast: a mechanobiological model. Women's Health. 2022;18:17455065221075907.

Douglas PS. Re-thinking benign inflammation of the lactating breast: classification, prevention, and management. Women's Health. 2022;18:17455057221091349.

Douglas PS. Does the Academy of Breastfeeding Medicine Clinical Protocol #36 'The Mastitis Spectrum' promote overtreatment and risk worsened outcomes for breastfeeding families? Commentary. International Breastfeeding Journal. 2023;18:Article no. 51 https://doi.org/10.1186/s13006-13023-00588-13008.

Guillen-Morales DdJ, Cruz-Cortes I, Sosa-Velazco TA, Aquino-Dominguez AS. The mother-infant symbiosis: a novel perspective on the newborn's role in protecting maternal breast health. Hygiene. 2025;5(46):https://doi.org/10.3390/hygiene5040046.