In a perfect world, a term, healthy newborn comes into the world vaginally. Again, I want to talk about normal. I know the process doesn't go normally all the time. (And I've talked about this here.)
The delivery of that baby close to the anus is critical for immune system development. The healthy, term newborn's gut is sterile (without bacteria) and the bacteria that get into that pristine gut are truly important. During a vaginal delivery, the largely harmless bacteria around the mother's anus are the bacteria getting into the newborn gut. They increase in number, compete for food and space and help coordinate efforts to create a healthy gut for that baby. With the exception of our skin, the gut is the largest immune system organ in our body.
Because breastfeeding is normal, what happens to healthy, term newborns who are breastfed is normal. The newborn has a delay in their immune response to bacteria. A delay? To a bacteria? Yup. Normal. After delivery, that newborn gut has many challenges from invaders that may not be friendly. Doesn't seem too smart not to fight back.
We all have mechanisms in our body to fight infection. In the gut it's called Gut Associated Lymphoid Tissue (GALT) and it's ready to roll at 19 weeks of gestation. All of the things that make up the GALT are waiting for a specific series of events to occur after delivery, when, if it proceeds normally, will result in a functioning immune system.
The sequence of those events is important. For example, after the good (commensal) bacteria has set up shop in the newborn gut, something called an "isolated lymphoid follicle" in the intestine of that baby develops. It's activated by substances in colostrum and helps with T cell development and function.
T cells are part of what is called the "innate" system. They mature in the thymus, an immune system structure found in the neck and chest of newborns. Human milk activates resting thymus cells, helping to shape the immune function of these cells. Breastfed kids have a larger thymus than those that are not breastfed; the thymus of the breastfed child is up to twice the size of a child not breastfed. The innate immune system contains cells that kill bacteria but they do it by also causing inflammation and tissue damage.
The innate immune system is different from the "adaptive" immune system, which is very specific to certain invaders. (Never being very good at immunology, but being really great at American football, I see the innate system as the offensive line, generally protecting from guys coming at the quarterback. The adaptive immune system is more like the wide receiver or cornerback- a player with a more specific job)
The cells of the adaptive immune system, antibodies, come in several flavors: Immunoglobulin M (IgM), which is the first type of antibody produced and isn't very specific; IgG which is transferred across the placenta and is the only immunoglobulin that the baby gets from mom and has at birth (the newborn, with only IgG is essentially immuno-compromised); IgE which isn't too relevant here; and IgA which rocks. IgA is a "sticky" immunoglobulin that protects surface areas from infection. A special type of IgA, secretory IgA, is found in huge numbers in human milk and protects the airway, gut and other mucous membranes from infection. Secretory IgA is special because the "secretory" part is an addition to the IgA and importantly, is resistant to being broken down by the baby' stomach and GI tract.
So, we have a new baby, exposed immediately to bacteria...why no inflammation? Well, the activity of the T-cells is delayed for about 10 days (remember, T-cells cause inflammation and tissue damage.) Well... secretory IgA helps. It's made by mom in response to infections in her environment and passed to the baby through breastfeeding. Moms and babies should stay together. This is one good reason: mom can't make antibodies to things that the baby is exposed to if the baby isn't with her.
Human milk also contains special sugars, oligosaccharides, which help feed good bacteria. In fact, they are necessary for that good bacteria to grow. Plus, they are a type of prebiotic- something can block bad bacteria before they ever get to the surface of the gut. They let the probiotics, the good bacteria, stay in the gut. And because they never let the bad bacteria get to the gut surface, no innate immune system is needed, and we get no inflammation or tissue damage. Oligosaccharides also work with certain receptors (called Toll Like Receptors). These receptors work in the first 5 days (when are our kids getting supplemented?) and are controlled tightly, like hour by hour.
In the time that the immune system is delayed, oligosaccharides, toll like receptors and good bacteria protect against bad bacteria and avoid the need for an inflammatory response. Any alteration in human milk or addition of formula interferes with toll like receptor function, changes the bacteria that the baby's gut gets exposed to and can then lead to inflammation and tissue damage, the result we were trying so hard to avoid.
Just one bottle.
The lesson? Let's make sure we know why we are supplementing.
(I suppose that's another soap box- check out the "3b's" birth weight, blood sugar and bilirubin)
Jenny Thomas, MD, IBCLC, FAAP, FABM
More info? A very nice book from Dr. Lars Hanson on Breastfeeding and Human Milk.