A type of white blood cell that recognizes and destroys infected or cancerous cells via its receptor.A T cell is a type of white blood cell that forms a central part of the adaptive immune system — the branch that learns to recognise specific threats and remembers them. Each T cell carries a unique receptor on its surface that fits a particular molecular shape, much like a lock and key. When that receptor encounters its matching target, the T cell activates and either kills the infected or cancerous cell directly or coordinates other immune cells. Because T cells can distinguish healthy tissue from abnormal cells with high precision, they are essential for controlling viral infections and for surveillance against tumours.
T cells were identified in the 1960s as lymphocytes that mature in the thymus, hence the 'T'. Their receptor was characterised in the 1980s, and the distinction between helper and cytotoxic T cells was established through work by several immunologists, including Jacques Miller and others.
T cells mature in the thymus, where those that react strongly against the body's own tissues are eliminated. A mature T cell circulates through blood and lymph, scanning the surfaces of other cells. It recognises fragments of proteins — called antigens — that are displayed on specialised molecules. Upon binding its target, the T cell receives additional co-stimulatory signals, then multiplies and differentiates into effector cells that fight the threat and memory cells that persist for years, ready to respond faster on a second encounter.
T cells are the basis of adoptive cell therapies such as CAR-T, in which a patient's own T cells are engineered to attack cancer, and they are the target of drugs that suppress rejection after organ transplantation.
The human body contains trillions of T cells, and their combined receptor diversity is estimated to be able to recognise more distinct molecular shapes than there are stars in the Milky Way.