The T cells of the adaptive immune system are created in the bone marrow by hematopoietic stem cells, but migrate to the thymus to mature. Both the stem cell population and the thymus decline with age, reducing the rate at which new immune cells arrive to take up the fight against pathogens and potentially cancerous cells. This reduced rate contributes to the age-related failure of the immune system, as misconfigured and damaged cells start to accumulate more rapidly than they can be replaced with fresh, functional cells. The open access paper here presents evidence to suggest that the effects of thymic decline are more subtle than simply an across the board reduction in the rate at which new T cells are supplied, however.

Both of these proximate causes of immune system aging might be addressed in the years ahead. There are several lines of research into thymic regrowth, such as through tissue engineering or delivery of FOXN1. Meanwhile the field of stem cell research should arrive at ways to invigorate old and declining stem cell populations, both replacing damaged cells with new cells, and reverting the stem cell niche changes that cause stem cells to become less active in later life.

Link: https://doi.org/10.1371/journal.pbio.2003352