The thymus was long considered as being inactive after puberty, but it continues to produce T lymphocytes until an advanced age. In a study (1) published in PLoS ONE, Vincent Geenen and his team demonstrated the importance of growth hormones in maintaining the production of T lymphocytes in adulthood.

The thymus is located in the thorax, behind the sternum and in front of the trachea. It could be called the "brain of the immune system". This primary lymphoid organ is located where nervous, endocrine and immune systems meet. It generates T lymphocytes and plays a crucial role in the auto-immune system - in other words, the body’s self-tolerance.

The immune system is made up of two large types of defence systems: innate (or natural) immunity and acquired (or adaptive) immunity. The former includes non-specific defence mechanisms such as physical barriers (skin, hair, mucus, ...) or chemical barriers (sweat, tears, saliva, gastric acid, urine,...), the phagocytic cells (macrophages, neutrophils, mastocytes ...) as well as the action of complement. These immune system components start working at birth and do not require early exposure to an aggressor in order to be triggered. They form the body's first line of defence and do so regardless of the nature of the infective agent.

Acquired immunity, on the other hand, is very specific. It enables the eradication of the aggressor, which is memorised after the first encounter. The main cells involved in this response are the T lymphocytes (thymo-dependent) and B lymphocytes which together enable recognition of the intrusive antigen, the destruction of infected cells and the production of antibodies to counter the unwanted attack.

The role of the thymus: educating the immune system

From an evolutionary perspective, “acquired immunity appeared around 500 million years ago with the emergence of cartilaginous fish. Almost simultaneously, a new organ was seen in these animals for the first time: the thymus. The thymus helps prevent any attack on the body’s adaptive immune system”, explains Professor Vincent Geenen from the ULg’s Immunoendocrinology Laboratory and Clinical Director of Endocrinology at the Liège CHU.

As they mature, T lymphocytes “cover themselves” with cellular receptors (TCR for T cell receptor), made up of membrane glycoproteins with an alpha chain and a beta chain. These antigen receptors are not identical: “As a result of genetic recombination, some parts of these receptors are variable", Vincent Geenen points out. This is what makes each T lymphocyte specific to a potential antigen.