What causes your breast to become inflamed when you're lactating?

Microscopic inflammation is a vital and constant (but invisible) process when your breasts are making milk

Sometimes, we get the impression that inflammation is bad. But inflammation is a basic protective process in the human body. Your mammary immune system is ancient, and protects both you and your baby during lactation.

Microscopic breast tissue inflammation, for example, is a vital and constant process when your breasts are making milk, which is healthy and good! This is why normal lactating breasts can be thought of as a 'pro-inflammatory' environment, which constantly activates wound-healing processes. Constant microscopic inflammatory processes form part of the healthy homeostasis of such highly dynamic tissue.

The microscopic inflammatory processes constantly at work in your lactating breast are your body's way of

1. Protecting you

• Regularly sweeping clean a highly dynamic tissue environment, where cells are constantly breaking down or building up. There will always be substantial cellular and other debri to clean up because of the high cell turnover during lactation.

• Removing any stray or rogue cells, which are more likely to take hold in a highly dynamic environment like the lactating breast, and cause disease down the track.

• Regulating interactions between your breasts' immune system and the microbiome of your breast milk, and also the microbiome of your breast tissue. Bacteria are a natural and healthy part of your breast and your breast milk. The microbiomes of your breast tissue and milk are an important part of your milk's immune superpowers. But these bacterial populations are constantly changing and adapting in response to your immune system and the breast's inflammatory processes. You can find out about your milk's microbiome here.

2. Dialling down your milk supply

Not only are invisible inflammatory processes a natural and healthy part of your breast’s immune activity, your breasts' inflammatory processes either turn down milk secretion when necessary, or turn milk secretion off completely when you wean.

If your breast's protective inflammatory processes become overactive, this shows up as a lump or engorgement

'Itis' means ‘inflammation of’. Colitis, for example, is a word which describes inflammation of the colon. Arthritis is an inflammation of the joint which causes problems or which can be detected by medical imaging. ‘Mast’ is from the Greek word 'mastos', which means breast. So mastitis really means any inflammation of the breast.

The term ‘mastitis’ really includes engorgement and (what are often called) 'blocked ducts', as well as the red hot painful areas we more commonly think about when we say mastitis!

But confusingly, our health system usually reserves the word mastitis for just one kind of breast inflammation - a red, hot lump, which might also be accompanied by fevers, muscles aches, feeling awful, and sometimes even uncontrollable shaking.

Your breasts will often be lumpy when they are working to make milk. An inflammatory lump, however, doesn’t go away after breastfeeding. Very importantly, any lump in your breast which doesn't disappear after a week needs to be assessed by your doctor.

You can find out why this is so important here.

What causes inflammation in your lactating breast to become overactive?

The main thing that sets off a cascade of worsening inflammation which becomes visible and painful in your breast is the development of an area of very high backpressures of milk within your milk glands and ducts.

High backpressures are caused by either

1. External mechanical compression of the milk ducts. Pressure gradually rises then in the milk glands as the amount of milk increases but is unable to be released through the ducts out into the baby's mouth or by leakage. Externally applied mechanical compression of the milk ducts is caused by

   • Nipple and breast tissue drag during breastfeeding, or

   • Other externally applied mechanical pressure, such as a poorly fitted garment, breast shell, or sleeping with pressure on the breast.

2. Producing much more milk than your baby needs, or spacing out breastfeeds.

Both external mechanical compression of the milk ducts and the pressure effects of more milk than baby needs or spacing out feeds results in stretching or even breakage of the junctions between the milk-making cells in a milk gland.

This triggers inflammation, which results in increased fluid and blood vessels in the stroma or connective tissue of the breast, which then further compresses or squashes the milk ducts and worsens the backpressure, in what we call an inflammatory cascade or cycle, which just gets worse and worse.

High backpressures can cause the kind of visible and tender or painful inflammation known as engorgement, when much of a breast or both breasts are affected.

Often, the inflammation is focussed in just one part of your breast, making the tissue swell to become a lump (commonly referred to as blocked ducts), perhaps with visible signs of redness and tenderness (commonly referred to as mastitis).

Recommended resources

Bad bugs and biofilm don't cause breast inflammation when you're lactating

How to care for your breasts when they're engorged

nastitis

Selected references

Anlas AA, Nelson CM. Interplay between hormonal and mechanical signals in mammary morphodynamics. Trends in Cell Biology. 2024;34(6):434-436.

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

Jindal S, Narasimhan J, Vorges VF, Schedin P. Characterization of weaning-induced breast involution in women: implications for young women's breast cancer. Breast Cancer. 2020;6(55):https://doi.org/10.1038/s41523-41020-00196-41523.

Kim T-J. Mechanobiology: a new frontier in biology. Biology. 2021;10(570):https://doi.org/10.3390/biology10070570.

Kobayashi K, Han L, Lu S-N, Ninomiya K, Isobe N, Nishimura T. Effects of hydrostatic ompression on milk production-related signaling pathways in mouse mammary epithelial cells. Experimental Cell Research. 2023;432:113762.

Paavolainen O, Peuhu E. Integrin-mediated adhesion and mechanosensing in the mammary gland. Seminars in Cell & Developmental Biology. 2021;114:113-125.

Quaglino A, Salierno M, Pellegrotti J. Mechanical strain induces involution-associated events in mammary epithelial cells. BMC Molecular and Cell Biology. 2009;10(55):https://doi.org/10.1186/1471-2121-1110-1155.

Stewart TA, Hughes K, Stevenson AJ, Marino N, Ju AL, Morehead M, et al. Mammary mechanobiology - investigating roles for mechanically activated ion channels in lactation and involution. Journal of Cell Science. 2021;134:doi:10.124/jcs.248849.