The act of breast-feeding conjures up images of tranquil bonding. It’s a special time for mothers and babies to snuggle close. And sure, sometimes it’s like that. But speaking from the trenches, I submit that there are some aspects that are not-so-picturesque. Sore nipples. Bite marks. And the latest case in point: Baby backwash.

That’s right, backwash, as in baby spit that gets slurped back into a nursing mother’s breast.

I recently heard about this backwash in an article in The Stranger about breast milk written by a food writer and nursing mother. In addition to describing breast milk’s palate-pleasing flavor profiles, the article details some of the immune factors that help keep breast-fed babies healthy. This is where the backwash comes in.

Part of the immunity that breast milk imparts, it seems, may depend in part on a mixture of milk and baby saliva flowing upstream. This backwash may actually cause a mother’s body to create made-to-order immune factors that are delivered back to the baby in milk, some scientists think. So far, that concept remains a hypothesis, cautions biologist Katie Hinde of Arizona State University in Tempe, “but one that remains very likely given all that we know about physiology.”

Scientists caught the backward flow of milk by watching as milk-associated fat globules moved from near the nipple to farther back in the breast in a 2004 ultrasound study. Although that retro flow hasn’t been directly observed while a baby feeds, it’s likely that the same thing happens during a nursing session.

Through this backwash, the baby may be placing an order that helps a mother’s body cook up special-ordered germ-fighting milk, cell biologist Foteini Kakulas (formerly Hassiotou) at the University of Western Australia in Crawley and colleagues believe.

In a series of experiments, Kakulas and her colleagues have found that mother’s milk rapidly changes in response to a baby’s infections. Breast milk usually contains a small number of infection-busting cells called leukocytes. When a baby (or a mother) is sick, the numbers of leukocytes in breast milk spike, Kakulas and colleagues reported in 2013 in Clinical and Translational Immunology.

Colostrum, an early form of milk that babies get in the first few days after birth, is packed with leukocytes. As babies get older, leukocytes in breast milk naturally wane if everybody’s healthy. When an infection appears, leukocyte levels come roaring back again. And this isn’t a trifling increase. During times of illness, a breast-feeding baby may gulp down billions of these cells a day, Kakulas and Donna Geddes, also of the University of Western Australia, estimated in a 2015 review in Advances in Nutrition. This leukocyte blast contains a mix of specialized cells that move from milk into the baby’s body. Some of these cells fight pathogens directly by gobbling them up or producing antibodies. Other cells come pre-programmed to fight germs that a mother had previously tangled with, transferring a memory that helps the baby’s immune system get established, animal studies have found.

Backwash may be one way that sick babies call for these leukocytes. Because this backwash relies on mouth-on-nipple action, that has implications for mothers who exclusively pump breast milk for their babies. Pumping moms may have a slightly blunted response to a baby’s infection, though that idea hasn’t been studied yet, Kakulas notes.

But pumping moms, don’t worry. Anyone who has spent time with a baby knows that there are lots of other opportunities for fluids — and information about germs — to be exchanged. As just one example, my baby drooled directly into my open mouth last week. Compared with the other ways babies get fluids into and onto their parents, baby spit backwash doesn’t even seem that disgusting. It’s just another way for my baby’s body to get a message into mine.