However, a significant fraction of lymphocytes that are produced by the body are auto-reactive. Auto-reactive T lymphocytes are lymphocytes that have receptors which recognise and target parts belonging to the body itself. It is therefore imperative to carry out a lymphocyte selection before they are released into the circulation. This selection is carried out by the thymus. In scientific terms, it is known as the thymic education process. In this way, only T lymphocytes which are not auto-reactive or are only slightly so, are most likely to recognise foreign antibodies, ignoring their differences. The others are destroyed.

A prolonged and little-known activity

Until recently, the prevailing idea was that the thymus operated only until puberty. This was because the size of the thymus reduces with age. “At the time, it was thought that from puberty onwards, the thymus gradually turned to fat, which was known as adipose involution of the thymus. This is, however, totally false,” points out the endocrinologist. Ten years ago, the development of a new technique enabled the opposite to be demonstrated: measuring TRECs, which are DNA excision circles produced through the rearrangement of genes encoding for the T antigen receptor in the thymus. “During genetic recombination of DNA segments encoding for the variable parts of the T lymphocyte antigen receptors, certain parts are separated and form what are known as excision circles", explains Gabriel Morrhaye, doctoral student at the Immunoendocrinology Laboratory and first author of the recent publication in PLoS ONE. “Using probes and quantitative PCR techniques, it is possible to quantify these excision circles and to gauge the thymus gland's activity", continues the student. Measuring TRECs enables T lymphocytes that emerge directly from the thymus to be identified in the blood. These are known as "recent thymic emigrants", and the level of their production can be measured. In the 1990s a group of US researchers used this technique to demonstrate that the thymus remained active until a very advanced age.