Breastfeeding and the Infant Thymus Gland
by Anne Eglash MD, IBCLC, FABM
If you have not seen an infant thymus gland on a chest x-ray, it is a large organ that looks like a huge blob in the center of the x-ray. It sits behind the sternum (breast bone), and between the lungs. It gradually shrinks over the course of childhood.
It is an organ that plays an important role in the development of the immune system by providing an environment for T lymphocyte cells to mature. Our bodies make B and T lymphocytes to fight disease processes such as infections.
Immature T lymphocytes are born in the bone marrow, and then migrate to the thymus, where they mature into fully functional T lymphocytes. The thymus is like a vocational school for immature lymphocytes to become trained soldiers in the immune system. Once the matured lymphocytes are released from the thymus, they go to live in the lymph nodes and have a role in fighting infections from invaders such as bacteria, viruses, and fungi.
In addition, the thymus gland plays a role in teaching tolerance to one’s own tissues. This helps to decrease the number of T cells that cause autoimmune diseases.
So, how does breastmilk alter or enhance the function/structure of the thymus gland? We know from a 1996 study that the thymus gland is larger in breastfed vs formula fed infants. Partially breastfed infants have an in-between sized thymus gland. As long as the infant is breastfed during the first year, the thymus gland remains larger and has greater lymphocyte output. The thymic enlargement from breastmilk gradually decreases after weaning, such that it is no larger than the thymus gland of formula fed infants approximately 2 weeks after weaning.
- Increased breastfeeding frequency has been found to be associated with increased CD4 T cell counts. (these are T cells that help coordinate immune response)
- A higher level of interleukin-7 in breastmilk correlates with larger thymus size. (interleukin -7 is a protein that helps mature T cells)
- Infants who are formula fed with a formula that has added interleukin-7 have similarly sized thymuses.
- Cortisol, a stress hormone in breastmilk, appears to play a role in infant thymus development.
- The breastfed infant’s gut bacteria, influenced by breastmilk, appears to secrete factors that enhance thymus development.
See the Answer
Breast feeding has been associated with improved infant outcomes in multiple aspects, including immune outcomes such as infections and potentially atopy and autoimmunity. However associations do not necessarily implicate cause and effect and at this point, exactly how breast feeding and components of breast milk may modulate the infant’s immune compartment remains unclear, especially in humans. Some lines of evidence suggest that breastfeeding affects the development of the infant’s thymus, a critical organ for T cell development. This may be a direct effect mediated by breast milk components or alternatively, a secondary effect from the impact of breast feeding on the infant’s gut microbiome. Here we discuss the potential mechanisms and impact of this association between breast feeding and thymic development.
There is no infant formula that has interleukin-7 added, so that was a trick question.
More information on breastmilk and thymus gland growth and activity comes from another recent study that evaluated breastmilk interleukin-7 levels and thymic gland development. They found that the more often the infants were breastfed, the higher the interleukin-7 levels in breastmilk, the larger the thymus size and CD3+ lymphocyte output. The partially breastfed infants had larger thymuses than the formula fed infants. A clinical use of this research is for those discussions about the value of partial breastfeeding. We can share that even partial breastfeeding enhances the function of the infant thymus, increasing its ability to mature healthy T cells and remove T cells that fight against self (auto-antibodies).