Illustration of a T cell (bottom) binding to a tumour cell (top). Antibodies (y-shaped) are attacking an immune checkpoint between the cells. T lymphocytes, or T cells, are a type of white blood cell and components of the body's immune system. They recognise a specific site (antigen) on the surface of pathogens, foreign objects, or tumour cells, bind to it, and attract antibodies or cells to eliminate that antigen. Healthy cells present molecules, known as immune checkpoints, to T cells to prevent them from being targeted. This is known as self-tolerance. Some cancer cells are able to take advantage of the immune checkpoints to prevent T cells from attacking them. These cancers can be treated with immune checkpoint inhibitors, a type of immunotherapy drug that attacks the checkpoint proteins, allowing the T cells to attack the cancer cells.