I participated in a study recently, in which the researcher was investigating the maternal cells present in breast milk. She needed fresh milk for the study, so I went to her lab to express some milk.

She provided a Medela Symphony breast pump for the mothers in the study, but I found that it didn’t really work for me to pump. Sweets is over 2.5 years old now and it’s been a year since I pumped at work. She typically only nurses 1-2 times per day, so I don’t have a lot of milk anymore, and the pump just wasn’t doing anything.

I decided to hand express instead, and I did get a little milk that way, but only about 6 mL. For reference, there are about 30 mL to an ounce, so that is really not very much milk!

The researcher let me watch as she centrifuged my milk sample, which separates the fat to the top of the sample and the cells to the bottom with the liquid portion of the milk in between. Then she removed everything except the cell pellet, washed the cells, and centrifuged them again. After the cells were washed, she looked at them on a microscope to count them. This is what we saw.

A photo of the cells from my breast milk sample

And from that little 6 mL, she was able to get 7,850,000 cells! That is 1.3 million cells per mL, which would be almost 40 million cells per oz! She had several experiments she wanted to do with the cells, and while this was enough for one experiment, it would not be enough for all of them. She encouraged me to come donate again if I was willing and able.

I went back two more times, with pretty similar results. The second time I had 6,160,600 cells in 5 ml of milk. The third time I had 9 million cells in 7 mL of milk.

Donation Total Cells mL cells/mL cells/oz
1 7,850,000 6 1,308,333 38,692,035
2 6,160,600 5 1,232,120 36,438,137
3 9,000,000 7 1,285,714 38,023,109
Average 1,275,389 37,717,760
Std Dev 39,142 1,157,559
Std Err 22,598 668,317

Not too bad for 2.5 years out! Next time someone tells you that there are no benefits to breastfeeding past a certain age, don’t believe it!  Most of the mothers who donated to the study had about half as many cells per mL than I did (though of course they had more milk volume too).

Another photo from my third milk sample

So the next question is, what are all those cells?

Most of the cells in milk are mammary epithelial cells.  The research study I donated to is investigating mammary stem cells in this subset.  Total leukocyte (white blood cell) counts are reported to be 4 million/mL in colostrum and 0.1-1 million/mL in mature milk. These are comprised of about 55-60% macrophages, 30-40% neutrophils, and 5-10% lymphocytes. Of the lymphocytes, about 80% are T cells and about 5% are B cells. Of the T cells, both CD8+ (cytotoxic) T cells and CD4+ (helper) T cells were present, and most were activated memory cells.

In short, a whole variety of live cells are present in breast milk. Milk is more than just a source of food for the infant. These cells can be absorbed by the infant through the mucosal membrane of the intestine, where they continue to carry out their normal functions. (One interesting side-effect of this is that it is postulated that kidney transplant recipients who were breastfed as infants and receive a half-matched kidney donated by their mother are less likely to experience rejection than those who were not breastfed, or those who receive a half-matched kidney donated by their father.) The maternal immune cells continue to  support and influence the child’s immune system.

It is an amazing gift that I give to my child, of my own body. Truly.