T cell
The immune system's trained assassins. Each T cell recognises one specific target displayed on a cell's surface and, if the cell is infected or cancerous, kills it. Most immunotherapy is about unleashing them.
T cells mature in the thymus and use a receptor (TCR) to inspect peptide fragments held in HLA molecules on other cells; cytotoxic CD8 T cells kill, CD4 helper cells coordinate, and regulatory T cells damp the response down. Tumours evade them by switching on checkpoints such as PD-L1, which exhausts the T cells that reach them, and by losing HLA so there is nothing to inspect. Checkpoint inhibitors reinvigorate exhausted T cells; CAR-T and TCR-T engineer a patient's own T cells with a new targeting receptor; bispecific T-cell engagers grab a T cell with one arm and a tumour cell with the other; and tumour-infiltrating lymphocyte (TIL) therapy expands the T cells already inside a tumour.