Why size matters when it comes to your baby's Thymus

If you have no clue what on earth a Thymus is, don't worry you aren't alone - but we all should, because despite it's striking lack of press it's a huge player in our body.

Thymus gland as sized at birth shown
by rubber model on baby's chest.

The key to a healthy, functioning immune system rests largely with the Thymus gland - a small organ lying just beneath the breast-bone. The Thymus increases gradually in size and activity until puberty, after which it begins to shrink and progressively dies off to be replaced by fat and connective tissue

It seems to do most of its work during the earlier part of life, removal in adulthood doesn't appear to increase the risk of auto immune disease; but it's an understudied area with lots of theorising.  Immunologist Kelley et al noted: "The involution (shrinkage) of the Thymus gland is one of the cardinal bio-markers of ageing" (1) with some suggesting the subsequent decline of Thymic hormones - gradually robs the body of its ability to fight off infectious diseases, autoimmune diseases, and cancer.

Regardless, what is known is the work done prior to it shrinking has lifelong impact.

The functions of the Thymus were not well understood until the early 1960s, when its role in the development of the immune system was discovered. Beginning during fetal development, the thymus entraps immature stem cells arising from the bone marrow and circulating in the blood. It then preprocesses these cells, causing them to become capable of maturing into a type of  lymphocyte (white blood cell); these migrate throughout the body via the bloodstream at various stages of maturity, seeding lymph nodes (ie giving rise to more lymph nodes, which act as filters or traps for foreign particles) and lymphatic tissue (tissue that contains large quantities of white blood cells).

The main cells undergoing this processing are T cells; some disrupt the membrane of the harmful bacteria, while others are involved in recognizing the “foreignness” and assisting a second sub-population of bone-marrow lymphocytes to respond. These killer cells and helper cells are essential to the regulation of immune responses and the development of cell-mediated immunity.

In fact they are so vital that if a newborn's thymus is removed, not enough T cells are produced and the the body's cell-mediated arm of immunity fails to develop.  It is this arm of immunity that is mainly responsible for rejection of organ transplants, resistance to microbial infection, and plays a role in the elimination of cells potentially able to give rise to cancer.

T cells are also destroyed in the thymus, in a process sometimes referred to as clonal purging i.e. only nonself-reactive cells leave the thymus. This process is believed to prevent autoimmune reactions.

Normally, by the time the infant is a few months old, the immune system has sufficiently formed so as to function throughout life. However, further growth and development of lymphoid tissue still depends on intervention by the thymic cells. After the initial seeding process, the thymus releases a hormonal substance that stimulates further growth of lymphoidal tissue, although this substance has not yet been isolated.

Why is this all relevant to your baby?
In 1996 a group of scientists decided to explore whether feeding method impacted on the growth of the Thymus during the first four months of life.

"At 4 months the geometric mean thymic index was 38.3 in exclusively breastfed infants, 27.3 in partially breastfed infants and 18.3 in formula fed infants. This finding was independent of weight, length, sex and previous or current illness. There was no significant difference in mean thymic index at birth between the three feeding groups and mean thymic index had increased in all three groups from birth to 4 months." (2)

They concluded:

"For the formula-fed infants it seems that the thymus remains large for a period and then decreases in size after breastfeeding has been terminated. The thymus is considerably larger in breastfed than in formula-fed infants at the age of 4 months." (2)

The above data indicates the Thymus is less than half the size in an exclusively formula fed infant.

The researchers then set about another study in 1999, to establish longer term impact:

"At 10 months the thymic index was significantly higher in those still being breast-fed compared to infants who had stopped breast-feeding between 8 and 10 months of age (P=0.05). This difference became more significant when controlled for the influence of infectious diseases (P=0.03). In infants still breast-fed at 10 months there was a significant correlation between the number of breast-feeds per day and their thymic index (P=0.01)" (3)

In 2004 scientists discovered another interesting fact:

"In rural Gambians, the season of birth strongly predicts adult mortality. Those born during the harvest season have longer life spans than do those born during the hungry season, and the deaths associated with infectious diseases suggest permanent early-life influences on immunity. Thymic measurements showed significantly smaller thymuses in infants born during the hungry season than in those born during the harvest season. The differences were greatest at 8 wk of age, a time when all infants were exclusively breastfed, which suggests the involvement of breast milk factors."

The objective of the study was to ascertain whether Thymic output was associated with breast milk interleukin 7 (IL-7) concentrations.They found:

"At 1 wk postpartum, the breast milk of mothers of infants born in the hungry season had significantly lower IL-7 than did that of mothers of infants born in the harvest season.  The findings were similar at 8 wk postpartum."

They concluded:

"These data suggests possible implications for long-term programming of immunity. "(4)

And I have to agree.  What is the long term cost to non breastfed infants in other countries of not receiving interleukin 7, what are the real implications of an undergrowth Thymus?  It reinforces that the impact of infant feeding is not limited to infancy or even childhood, but far beyond.  We have merely scratched the surface in discovering what is and isn't a vital constituent of breastmilk, and indeed exactly which areas of health this can influence; until then it seems to me, on some level infant feeding is still one big experiment.

1. K. Kelly et al. 'A pituitary-Thymus Connection During Aging.' Ann. N.Y. Acad. Sci. 521, 88-98
2. Decreased thymus size in formula-fed infants compared with breastfed infants, EUROPEAN JOURNAL OF PEDIATRICS
Volume 158, Number 12, 964-967, DOI: 10.1007/s004310051258
3. Breast-feeding influences thymic size in late infancy, EUROPEAN JOURNAL OF PEDIATRICS, Volume 158, Number 12, 964-967, DOI: 10.1007/s004310051258
4. Improved thymic function in exclusively breastfed infants is associated with higher interleukin 7 concentrations in their mothers' breast milk. American Journal of Clinical Nutrition, Vol. 80, No. 3, 722-728,