Cytokine
Small proteins crucial for immune signaling and inflammation.
Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. They are produced by a broad range of cells, including immune cells, endothelial cells, fibroblasts, and various types of connective tissue cells. Cytokines are especially important in the immune system, including in immune responses and inflammation, and they modulate the balance between humoral and cell-based immune responses.
- category
- Small proteins (~5–25 kDa)
- function
- Cell signaling, especially in immune responses and inflammation
- produced_by
- Broad range of cells, including immune cells, endothelial cells, fibroblasts, and connective tissue cells
- includes
- Chemokines, interferons, interleukins, lymphokines, and tumour necrosis factors
- mechanism
- Interact with specific cytokine receptors on target cell surfaces
- signaling_types
- Autocrine, paracrine, and endocrine
Lore & Background
The term 'cytokine' derives from ancient Greek: 'cyto' (cell) and 'kines' (movement). Cytokines have been classed as lymphokines, interleukins, and chemokines based on presumed cell of secretion, function, or target, but such distinctions are now considered obsolete due to redundancy and pleiotropism. The term 'interleukin' was initially used for cytokines targeting white blood cells, but now bears little relation to presumed function; most are produced by T-helper cells.
Reader's Guide
Cytokines are fundamental to immune system function, regulating both humoral and cell-based responses. They are released in lower concentrations than hormones and can act locally or systemically. Their classification includes structural families (e.g., four-α-helix bundle, IL-1, cysteine knot, IL-17) and functional types (type 1 enhancing cellular immunity, type 2 enhancing antibody responses). Dysregulation of cytokine balance is studied in autoimmune disorders. Cytokine receptors, including those in the immunoglobulin superfamily and hemopoietic growth factor family, are critical for understanding immunodeficiency states like X-linked Severe Combined Immunodeficiency. Cytokines also play roles in anti-inflammatory pathways and are potential therapeutic targets for pathological pain.
Did You Know?
- Cytokines are usually too large to cross cell membranes and function by interacting with specific receptors on target cells.
- The IL-2 receptor's γ-chain deficiency is directly responsible for X-linked Severe Combined Immunodeficiency (X-SCID).
- Chemokine receptors include two that act as binding proteins for HIV (CD4 and CCR5).
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