The white cell count in your breast milk increases dramatically when you have detectable breast inflammation

Together, white cells from your immune system and epithelial cells from the lining of your milk ducts make up what we call the 'somatic' cells of your milk. Somatic cells are quite different to the microorganisms which are found in your milk microbiome.

Your milk's white cells secrete protective factors. If you develop an inflammation of your breast, whether it is painful nipples or blocked ducts or mastitis, the amount of white cells found in your milk increases exponentially.

• Normally, white cells are about 5% of the cells which are found in your milk.

• But when your breast becomes detectably inflamed, white cells make up about 95% of the cells found in your milk.

White cell concentrations in your milk stay high until the mastitis has healed. Milder breast conditions such as sore nipples or blocked ducts have less dramatic white cell rises, but the white cell count still does increase.

High white cell counts affect your milk's microbiome when you have breast inflammation

The complex ecosystem of human milk adapts under stress (such as the mechanical stress caused by very high backpressures in the milk glands), with multiple factors including the milk metabolome, oligosaccharides, and white cells trying to restore equilibrium through upregulation of some feedback loops and downregulation of others.

In your milk during a bout of detectable breast inflammation, as well as having very high white cells counts

• Your total milk bacterial counts climb and the diversity of bacteria in your milk microbiome decreases. This is not surprising, because white cells destroy bacteria and secrete anti-microbial factors. In such high numbers, they will start culling the less resilient bacterial types from the microbiome. This is a side-effect (not a cause) of inflammation.

• Some bacteria increase in numbers, like Staphylococcus. Staphylococcus aureus, for example, is a very common bacteria which exists in the human body and in your milk mostly without causing any trouble. It is however very well adapted in human environments and more resilient than many other bacteria in the face of high white cell counts. This doesn't mean that we need to treat it with antibiotics, because it is only in perhaps a third of cases of mastitis that the presence of S. aureus becomes so dominant that antibiotics help with recovery.

This is why, if you're lactating and develop breast inflammation, we need to support, before all else, the resilience of your immune system's feedback loops, by relieving milk backpressures and opening up the ducts with milk ejections or let-downs.

Unfortunately, you might be advised to do a lot of things which either don't help, or may actually make the breast inflammation worse. Elsewhere, I discuss this unhelpful advice, show why it isn't science based, and what we can do instead.