We now know that T cells, a critical component of the immune system, mature in the thymus. It is here that they learn a crucial lesson: the thymus acts as a quality control centre, allowing T cells that recognise foreign pathogens to survive while eliminating those that would attack the body’s own tissues. This process of weeding out self-reactive T cells is termed ‘central tolerance’.

While studying this process in the late 1960s and 1970s, scientists observed something peculiar: Removing the thymus in newborn mice not only weakened their ability to fight infections but, paradoxically, often led to autoimmune diseases, in which the body’s own T cells attack its tissues.

The timing of the removal, however, produced a dramatic difference. Removing the thymus on day 3 after birth led to se

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