The female breast or mammary gland evolved from ancient mammalian immune systems

Way back in Deep Time - perhaps 310 million years ago, according to evolutionary biologists - our ancestral mammary gland was a modified oil gland in the skin. A primordial ancestor's body secreted a potent mucous saturated with immune superpowers to protect her newborn. The collection of cells which secreted this powerful fluid became what we call her mammary gland. This ancient grandmother became the first mammal.

Evolutionary biologists think that the special sheltering fluid she made for her baby only developed into the caloric-rich nutrition which we call milk much later on down the evolutionary journey. In light of the extraordinarily protective powers of human milk being uncovered by contemporary lactation science, it makes complete sense that our astonishing milk-making capacity grew out of ancient ancestral immunity.

Thinking about the evolutionary origins of female human breasts and milk is important for two reasons. For a start, it's a reminder that your breast milk is a living extension of your own immune superpower, deposited into your baby’s mouth and gut. From there, your precious gift of protection blossoms right throughout your baby’s blood stream and lymph, tissues and cells.

But very importantly, understanding the immune origins of the human breast and milk is helpful when you experience a problem such as engorgement, plugged or blocked ducts or mastitis. The process of making milk actually draws on the protective, healing functions of your body’s immune responses.

So before all else, from the dawn of time, our lactating Homo sapiens breasts has brought the human baby into the shelter of the maternal body's immune ecosystem, washing every cell of that nascent little human in a pulsing network of chemical and cellular protection.

Your lactating breasts are meant to have a lot of healthy, invisible inflammatory (or 'wound-healing') activity

At the heart of the healing powers of your immune system is the process known as inflammation. Milk-making breasts have a constant low level of inflammation at work (lactating breasts are a pro-inflammatory or wound-healing environment), day and night, to keep your breasts healthy and safe during this time of dynamic generative change. If you know that breast inflammation during lactation is simply the protective power of your breasts kicking in, conditions such as engorgement, 'blocked ducts', and mastitis seem less frightening. This knowledge also leads to more effective treatments.

Your working breasts radiate protective power, regardless of whether your breasts are very delicately shaped, like mine, or wonderfully generous and resting upon your tummy, or somewhere in between. But the magnificent symphony of your breasts’ immune superpower doesn’t have a single conductor. Instead, this protective power self-regulates through interactions between hundreds of thousands or maybe millions of feedback loops, each part listening to its neighbour and responding.

This astonishing self-regulating symphony is played out by four key biological systems which interact together through a multitude of dynamic molecules and cells and tissues in your breasts, many of which we don’t yet understand. Instead of strings, woodwinds, brass and percussion, your lactating breasts’ immune power is made up of

1. The local immune cells which inhabit your breast stroma

2. Your breasts’ mechanical or pressure sensing capacities

3. Your milk, which itself contains multiple interacting and protective biological systems, including your

   • Milk's microbiome

   • Milk's metabolome, powerfully shaped by the microbiome

   • Milk's somatic cells

   • Milk's oligosaccharides; all nested in

4. Your body’s greater immune system, including the formidable generative powers of your bone marrow, gut and lymph nodes. The many elements of your immune system are brought into your breast by the blood stream as required.

Human milk is a living tissue which contains living (and dead) somatic cells, as well as the microbiome

As well as a microbiome, human milk also contains what we collectively call somatic cells. There are about 10,000 of these living cells (which don’t include the microorganisms of your milk microbiome!) in each microlitre of your milk. These cell populations in human milk are also highly dynamic and variable.

The population of somatic cells is made up of

• Luminal cells (predominantly lactocytes), the kind of mammary epithelial cells which line the alvoli and lactiferous ducts, which are regularly shed. Most of these luminal cells in your milk are lactocytes. They still secrete milk proteins once they've fallen off their basement membranes and are floating freely in your milk!

• Myoepithelial cells, the kind of mammary epithelial cells which hug the basement membrane and wrap star-like round the luminal cells.

• Protective white cells. There are high numbers of leucocytes in colostrum, but they fall by four-fifths to comprise just 2% of cells in mature milk. But when the breast is stressed with a mastitis or visible inflammation, the white cells increase to 95% of the milk cells. They also increase in numbers, but less so, with painful nipples or with blocked ducts

• Up to 6% of these cells are stem cells, which have the capacity to repair tissue by developing into a range of cells, perhaps mostly turning into lactocytes and myoepithelial cells.

Recommended resources

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

Selected references

McClellan HL, Miller SJ, Hartmann PE. Evolution of lactation: nutrition v. protection with special reference to five mammalian species. Nutrition Research Reviews 2008;21:97-116.

Oftedal O. The evolution of milk secretion and its ancient origins. Animal 2012;6(3):355-368.