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As scientists learned more, it was understood that these proteins and others were part of a broader class of proteins involved in self-defense, and should be called "cytokines".M) concentrations that can increase up to 1,000-fold during trauma or infection.The word comes from Greek: cyto, from Greek "κύτος" kytos "cavity, cell" kines, from Greek "κίνησις" kinēsis "movement".In 1969 Dudley Dumonde proposed the term "lymphokine" to describe proteins secreted from lymphocytes and later, proteins derived from macrophages and monocytes in culture were called "monokines".They are different from hormones, which are also important cell signalling molecules, in that hormones circulate in less variable concentrations and hormones tend to be made by specific kinds of cells.

Dysregulation of this tendency is under intensive study for its possible role in the pathogenesis of autoimmune disorders.Cytokines include chemokines, interferons, interleukins, lymphokines, and tumour necrosis factors but generally not hormones or growth factors (despite some overlap in the terminology).Cytokines are produced by a broad range of cells, including immune cells like macrophages, B lymphocytes, T lymphocytes and mast cells, as well as endothelial cells, fibroblasts, and various stromal cells; a given cytokine may be produced by more than one type of cell.Such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.

Each cytokine has a matching cell-surface receptor.The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type.