Either of two proteins that act as cytokines, eliciting numerous responses in virtually all types of cells by activating cellular signalling pathways and promoting the expression of genes. They are involved in many aspects of the body’s immune defences, notably in their ability to destroy tumour cells; they also assist in the killing of virally infected cells by natural killer cells. TNF-α‎ is a glycoprotein produced chiefly by monocytes and macrophages, whereas TNF-β‎ (or lymphotoxin) is produced by T cells. Both bind to either of the cell-surface receptors—tumour necrosis factor receptors I and II (TNFR-I and TNFR-II). These proteins give their name to a family of related proteins that typically regulate aspects of immune responses and cell death.