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Illustration of a killer T cell (left) with a T cell receptor (upper) and CD8 (cluster of differentiation 8) co-receptor (lower) on its surface and a helper T cell (right) with a T cell receptor (upper) and a CD4 (cluster of differentiation 4) co-receptor (lower) on its surface. T cells, or T lymphocytes, are a type of white blood cell and one of the components of the body's immune system. Their receptors and co-receptors recognise specific sites (antigens) on the surface of cells or pathogens and bind to them. Helper T cells then signal for other immune system cells to eliminate the cell. Killer, or cytotoxic, T cells eliminate the cell themselves by releasing a protein that forms pores in the cell's membrane. For a labelled version of this image see F044/8781