Cytokines: Meaning, Functions and FAQs

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What are Cytokines? – Cytokines Meaning

Cytokines are small immunomodulating proteins that range from 5-25 kDa. They are peptide molecules that are important for cell signalling but cannot cross the lipid bilayer of the cell to enter the cytoplasm. They are involved in paracrine, autocrine and endocrine signalling. Cytokines should not be confused with hormones. Hormones are a more specific kind of molecule as compared to cytokines. Read further to get an overview of cytokines.

Cytokines are produced by various cells such as macrophages, mast cells, B-lymphocytes, T-lymphocytes, fibroblasts as well as endothelial cells, among others. The cytokines produced by lymphocytes are lymphokines, monocytes are monokines, chemotactic activities are chemokines, and leukocytes are interleukin. Basically, cytokine is a general term that is used to describe protein molecules that are important for immune system regulation and intercellular communication.

Properties of Cytokines

  • Cytokines may act on the cell that produces them (autocrine action), on the nearby cells (paracrine action) or on distant cells (endocrine action).
  • They are pleiotropic in nature, meaning that a single cytokine can act on different cell types, or different cell types may secrete the same cytokine.
  • Different cytokines can stimulate similar functions, meaning they are redundant in nature.
  • They are mostly produced in cascades, as one cytokine estimates the release of other cytokines.
  • Cytokines can work both antagonistically and synergistically.
  • Each cytokine has a matching cell receptor, and it can either upregulate or downregulate the activity of genes.

Classification of Cytokines

The cytokines are divided on the basis of their structure and function. Let us look at them in detail.

Structural Classification

The structural homogeneity of the cytokines helps to differentiate them into four families:

  1. The four-α-helix bundle family consists of cytokines that have three-dimensional structures and a bundle of four α-helices. They have three subfamilies:
    • IL-2 subfamily
    • Interferon (IFN) subfamily
    • IL-10 subfamily
  2. The IL-1 family includes IL-1 and IL-18.
  3. The cysteine knot family consists of transforming growth factor beta superfamilies such as TGF-β1, TGF-β2 and TGF-β3.
  4. The IL-17 family includes the IL-17 cytokines.

Functional Classification

The functional classification of the cytokines divides it into two types:

  1. Type 1 includes TNFα, IFN-γ, etc., which enhance cellular immune responses.
  2. Type 2 includes IL-4, IL-10, TGF-β, IL-13, etc., which enhance antibody responses.

Function of Cytokines

  • Cytokines are majorly produced by helper T cells and macrophages. During a peripheral nerve injury, the employed macrophages, mast cells and endothelial cells produce cytokine at the site of injury for nerve regeneration.
  • Cytokines such as IL-6 and TNF-α are involved in the upregulation of inflammatory reactions.
  • IL-1β released by monocytes and macrophages are released at the time of inflammation, injury, invasion and infection.
  • The chemokines function for the activation and migration of leukocytes. They are also released during demyelinating and neuroinflammatory diseases.
  • Various cytokines are also involved in modulating the neuronal activity of the neurons of the central and peripheral nervous systems.

Receptors of Cytokines

The receptors of cytokines have been divided into six families based on their three-dimensional structures:

  1. Immunoglobulin (Ig) superfamily that is similar to immunoglobulins. IL-1 cytokines bind to such receptors.
  2. Hemopoietic Growth Factor (type 1) family that have conserved motifs in their extracellular amino acid domain. IL-2 cytokines bind to such receptors.
  3. The interferon (type 2) family binds to IFN-γ and IFN-β.
  4. Tumour necrosis factor (TNF) type 3 family has cysteine-rich extracellular domains that bind to TNF cytokines.
  5. Seven transmembrane helix families are ubiquitous receptors of animals.
  6. The interleukin-17 receptor (IL-17R) family binds to IL-17 cytokines.

Hormones vs Cytokines

Cytokines are small proteins that are secreted by immune and non-immune cells. On the other hand, hormones can be proteins, steroids, amino acid derivatives or fatty acid derivatives that are secreted by glands. Cytokines bind to receptors to modulate cell growth and immune responses, whereas hormones control homeostasis, development and reproductive functions.

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Frequently Asked Questions

Q1

What are the three functions of cytokines?

Cytokines are involved in nerve regeneration, immune response and upregulation of inflammatory reactions.
Q2

What produces cytokines in the body?

Cytokines are produced by both immune and non-immune cells such as macrophages, T helper cells, fibroblasts and endothelial cells.
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