Keep Breastfeeding: 5 Facts about Secretory IgA

human milk composition secretory iga Feb 13, 2023

My daughter’s best friend, who lives in Salt Lake City, recently texted me that she was sick with Covid and wondered if she should stop breastfeeding her 3-month-old. My niece, who lives in upstate New York, recently called when she was diagnosed with a bad GI bug and wondered if it was safe to keep breastfeeding her 4-month-old little boy. My daughter-in-law recently called to share that she tested positive for the respiratory syncytial virus (RSV) and was concerned about breastfeeding her 5-week-old (my granddaughter!)

In each case I gave the same advice which was to wash hands, wear a mask, and, if the she felt well enough, KEEP BREASTFEEDING. Stopping breastfeeding would be a dangerous thing to do for the baby. This blog is about the science behind these recommendations.

1) Main antibody

The human body makes five classes of antibodies or immunoglobulins (Ig for short): IgG, IgM, IgA, IgD and IgE. An antibody is a protein. The main antibody in human milk is immunoglobulin A (IgA), in the form of secretory IgA (sIgA).

2) Secretory IgA

sIgA is a special form of IgA that is not digested by the acid in the stomach or any of the enzymes in the baby’s gut. Secretory IgA is composed of two IgA molecules, a joining protein called the J chain, and a secretory component. It is the secretory component that protects the IgA antibody from being destroyed in the gut.

Below is a depiction of sIgA showing the two IgA molecules, a connecting J chain, and a secretory component. Antigens (like viruses) are bound on the ends. (1)

3) High amounts in colostrum

sIgA is especially high in colostrum in the first 72 hours after birth. Levels then drop a bit but are maintained in human milk throughout the first four months and even up to a year.

4) Varnish defense

The body protects itself from foreign invaders in several ways. One way is a system involving the spleen and the lymphatic system in conjunction with the blood. Another way is via the mucous that lines cells in the gut, respiratory tract and urogenital areas. IgA is part of this defense system. It is localized in these mucosal linings, ready to do battle with almost any pathogen that it comes in contact with.

I think of secretory IgA as being like a varnish that is painted on the lining of the gut. It grabs onto pathogens, blocks them from entering the body, then releases from the lining, carrying them out of the body. Studies have found secretary IgA in the baby’s feces unaltered, indicating it is not absorbed in the gut.

5) Enteromammary pathway

Let’s get back to my niece – she is the breastfeeding mother who suddenly felt sick with stomach pains. She vomited a few times and developed a fever and diarrhea. She took my advice and kept breastfeeding. She recovered in several days and her baby never got sick.

Was this just a coincidence or did her breast milk contain special protection against that specific virus?

A fascinating system comes into play that protects the infant. Let’s walk through it.

  • The GI virus gets into the mother’s GI tract. (She gets sick.)
  • Special cells, called M Cells, are located among the cells that line the GI tract.
  • The M cells notice the virus (what are you doing here?), grab onto it, and obtain a sample of the virus.
  • Other types of cells arrive to work with the M Cells, helping to get the viral information to a cluster of cells, called Peyer’s Patches, that are located just beneath the gut lining.
  • Within the Peyer’s Patches, the information about the virus is passed on to another special cell, called a plasma cell. A plasma cell is a type of lymphocyte, which is a type of white blood cell.
  • The plasma cell – now with the information about the virus - is then on a mission. It leaves the Peyer’s Patch, enters the lymphatic system, is dumped into the blood stream, and then arrives at the breast tissue.
  • In the breast tissue, the plasma cell produces an antibody, our friend, immunoglobulin A (IgA). Keep in mind that this IgA has specific protection against the virus that was causing the mother’s diarrhea.
  • One more change occurs, this time converting the IgA into secretory IgA (sIgA) which we have discussed. I am not exactly sure how IgA transforms into secretory IgA but it does happen!

The baby drinks from the breast. The milk contains a large amount of this specific secretory IgA. The milk containing the sIgA makes its way to the baby’s intestines and settles into mucous lining the walls. There it can look for and grab onto any of the diarrhea-causing virus, preventing it from attaching to and entering the infant’s intestinal cells. It holds onto it and is excreted in the infant’s poop.

And almost like magic, the baby never gets sick. Gut to breast to milk is the Enteromammary System.

That is why the recommendation in each case was Keep Breastfeeding!


  1. Image from the British Society of Immunology, Immunoglobulin A (IgA). British Society for Immunology, Bite-sized Immunology. Rhonda Curran, Queen’s University Belfast, UK

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