Tumor Necrosis Factor (TNF), also known as TNFα, is a kind of cytokine that can kill tumor cells directly and has no obvious cytotoxicity to normal cells. TNF is a cell signaling protein involved in systemic inflammation, and it is one of the cytokines that constitute the acute phase response. It is produced primarily by activated macrophages, but most other immune cells, such as CD4+ lymphocytes, NK cells, neutrophils, mast cells, eosinophils, and neurons in the nervous system can also produce it.