What do monocytes, lymphocytes, and neutrophils all have in common? Well, yes, they are all leucocytes and part of our immune system, but what else? They all can be prompted to migrate to the site of infection by a specific class of cytokines known as chemotactic cytokines, or chemokines for short. These chemokines are made by a wide variety of cell types, including macrophages, various blood cells, epithelial cells, endothelial cells, and, of particular interest here, by numerous human tumor cell lines, melanoma cells, and liver cells infected by hepatitis C virus. Some chemokines function to direct cell migration during normal processes, such as embryogenesis, lymphoid organ development, and haematopoiesis. The chemotactic responses they incite can be quite specific, with different cell populations responding to different chemokines, of which there are more than 40, classified into four families. They’re all small proteins (~8–10 kDa), all have the same characteristic structure, and all are homologous, with 20–50% amino acid identity. That much divergence in sequence provides plenty of opportunity for specificity of action.
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