Any of a large family of small proteins, produced by many types of cells, that attract and guide white blood cells (leucocytes) to sites of infection or influence lymphocyte development and migration in lymph nodes. Hence they are a crucial aspect of innate immunity during the initial response to injury or pathogen invasion and affect lymphocyte behaviour in adaptive immune responses. They fall into four main structural categories: C, CC, CXC, and CX₃C, according to the number of cysteine residues (C) or variable amino acids (X) near the amino terminus of the protein. For example, CXCL8 (formerly interleukin 8) is produced by monocytes, macrophages, and other cells and causes neutrophils to migrate from nearby blood vessels to the site of inflammation. CCL2 similarly attracts monocytes from the bloodstream and induces them to change into tissue macrophages. Chemokines act in conjunction with other factors, such as tumour necrosis factor α‎ (TNF-α‎), which induces the adhesion factors necessary to bring the leucocytes alongside the blood vessel wall before they ‘squeeze through’ into the tissues.