Interleukins are a group of signaling molecules, or cytokines, that play important roles in cell communication within the immune system.

They are produced by various cells, including white blood cells, and are involved in regulating immune responses and inflammation.

Interleukins help coordinate the body’s defense against pathogens, promote the activation and proliferation of immune cells, and regulate the duration and strength of immune responses.

The human genome encodes more than 50 interleukins and related proteins.

The function of the immune system primarily depends on interleukins, and rare deficiencies of a number of them have been described, all featuring autoimmune diseases or immune deficiency. 

The majority of interleukins are synthesized by CD4 helper T-lymphocytes, as well as through monocytes, macrophages, and endothelial cells. 

Interleukins promote the development and differentiation of T and B lymphocytes, and hematopoietic cells.

Some interleukins are classified as lymphokines, lymphocyte-produced cytokines that mediate immune responses.

Interleukin 1 alpha and interleukin 1 beta (IL1 alpha and IL1 beta) are cytokines that participate in the regulation of immune responses, inflammatory reactions, and hematopoiesis.

Two types of IL-1 receptor, each with three extracellular immunoglobulin (Ig)-like domains, limited sequence termed type I and type II receptors.

The receptors can both bind all three forms of IL-1, IL-1 alpha, IL-1 beta and IL-1 receptor antagonist.

Interleukin 1 also plays a role in the central nervous system.

Interleukin 2

T lymphocytes regulate the growth and differentiation of T cells and certain B cells through the release of secreted protein factors, which include interleukin 2 (IL2).

IL2 is a lymphokine that induces the proliferation of responsive T cells. 

In addition, it acts on some B cells, via receptor-specific binding, as a growth factor and antibody production stimulant.

Interleukin 3

Interleukin 3 (IL3) is a cytokine that regulates hematopoiesis by controlling the production, differentiation and function of granulocytes and macrophages.

The protein, which exists in vivo as a monomer, is produced in activated T cells and mast cells, and is activated by the cleavage of an N-terminal signal sequence.

IL3 is produced by T lymphocytes and T-cell lymphomas only after stimulation with antigens, mitogens, or chemical activators such as phorbol esters. 

However, IL3 is constitutively expressed in the myelomonocytic leukemia cell line.

It is thought that the genetic change of the cell line to constitutive production of IL3 is the key event in development of this leukaemia.

Interleukin 4

Interleukin-4 (IL-4) is a cytokine that is primarily produced by activated T cells, mast cells, and basophils. 

It plays a critical role in regulating the immune response, specifically in promoting an immune response known as the Th2 response.

Interleukin 4 (IL4) is produced by CD4+ T cells specialized in providing help to B cells to proliferate and to undergo class switch recombination and somatic hypermutation to the IgG1 and IgE isotypes.

IL-4 stimulates the differentiation of naive T cells into Th2 cells, which are a subset of T helper cells that are involved in antibody production and immune responses against parasites and allergens. 

IL-4 also promotes the growth and activation of B cells, leading to the production of immunoglobulin E (IgE), which is important in allergic responses.

Furthermore, IL-4 is known to inhibit the production of certain cytokines involved in the Th1 response, which is associated with cellular immunity and defense against intracellular pathogens. 

This suppression of Th1 cytokines helps to maintain the balance between Th1 and Th2 responses in the immune system.

IL-4 is implicated in promoting tissue repair processes, such as wound healing and tissue remodeling, and can contribute to the development of certain diseases, such as asthma and allergic reactions.

Overall, interleukin-4 plays a crucial role in modulating the immune response, promoting allergic reactions, stimulating the production of antibodies, and influencing tissue repair processes. Its functions are carefully regulated and its dysregulation can contribute to immune disorders and allergic diseases.

Interleukin 5 (IL5), also known as eosinophil differentiation factor (EDF), is a lineage-specific cytokine for eosinophil production.

It regulates eosinophil growth and activation.

It has an important role in diseases associated with increased levels of eosinophils, including asthma.

Interleukin 6

Interleukin 6 (IL6), also referred to as B-cell stimulatory factor-2 (BSF-2) and interferon beta-2, is a cytokine involved in a wide variety of biological functions.

It has an essential role in the final differentiation of B cells into immunoglobulin-secreting cells, as well as inducing myeloma/plasmacytoma growth, nerve cell differentiation, and, in hepatocytes, acute-phase reactants.

A number of other cytokines may be grouped with IL6 on the basis of sequence similarity.

These include granulocyte colony-stimulating factor (GCSF) and myelomonocytic growth factor (MGF). 

GCSF acts in hematopoiesis by affecting the production, differentiation, and function of 2 related white cell groups in the blood.

MGF also acts in hematopoiesis, stimulating proliferation and colony formation of normal and transformed avian cells of the myeloid lineage.

Cytokines of the IL6/GCSF/MGF family are glycoproteins of about 170 to 180 amino acid residues that contain four conserved cysteine residues involved in two disulphide bonds.

Interleukin 7 (IL-7) is a cytokine that serves as a growth factor for early lymphoid cells of both B- and T-cell lineages.

Interleukin 8 is a chemokine produced by macrophages and other cell types such as epithelial cells, airway smooth muscle cells and endothelial cells. 

Endothelial cells store IL-8 in their storage vesicles, the Weibel-Palade bodies.

The interleukin-8 protein is encoded by the CXCL8 gene.

IL-8 is initially produced as a precursor peptide of 99 amino acids which then undergoes cleavage to create several active IL-8 isoforms.

There are many receptors on the surface membrane capable of binding IL-8.

The  most frequently studied types are the G protein-coupled serpentine receptors CXCR1 and CXCR2. 

Expression and affinity for IL-8 differs between the two receptors (CXCR1 > CXCR2). 

IL-8 is secreted and is an important mediator of the immune reaction in the innate immune system response.

Interleukin 9 (IL-9) is a cytokine that supports IL-2 and IL-4 independent growth of helper T cells. 

Interleukin 10 (IL-10) is a protein that inhibits the synthesis of a number of cytokines, including IFN-gamma, IL-2, IL-3, TNF, and GM-CSF produced by activated macrophages and by helper T cells. 

IL-10 is highly similar to the Human herpesvirus 4 (Epstein-Barr virus).

Interleukin 11 (IL-11) is a secreted protein that stimulates megakaryocytopoiesis, initially thought to lead to an increased production of platelets.

 It is redundant to normal platelet formation.

It activates osteoclasts, inhibiting epithelial cell proliferation and apoptosis, and inhibiting macrophage mediator production. 

These functions help mediating the hematopoietic, osseous and mucosal protective effects of interleukin 11.

Interleukin 12 (IL-12) is involved in the stimulation and maintenance of Th1 cellular immune responses, including the normal host defense against various intracellular pathogens, such as Leishmania, Toxoplasma, Measles virus, and Human immunodeficiency virus 1 (HIV). 

IL-12 also has an important role in enhancing the cytotoxic function of NK cells and role in pathological Th1 responses, such as in inflammatory bowel disease and multiple sclerosis. 

Suppression of IL-12 activity in such diseases may have therapeutic benefit. 

The administration of recombinant IL-12 may have therapeutic benefit in conditions associated with pathological Th2 responses.

Interleukin 13 (IL-13) is a pleiotropic cytokine that may be important in the regulation of the inflammatory and immune responses.

IL-13 inhibits inflammatory cytokine production and synergies with IL-2 in regulating interferon-gamma synthesis. 

Interleukin 15 (IL-15) is a cytokine that possesses biological functions, including stimulation and maintenance of cellular immune responses.

IL-15 stimulates the proliferation of T lymphocytes, which requires interaction of IL-15 with IL-15R alpha and components of IL-2R, including IL-2R beta and IL-2R gamma.

Interleukin 17 (IL-17) is a potent proinflammatory cytokine produced by activated memory T cells.

This cytokine IL-17  is characterized by its proinflammatory properties, role in recruiting neutrophils, and importance in innate and adaptive immunity. 

IL-17 plays a key role in inflammation of many autoimmune diseases, such as RA, allergies, asthma, psoriasis, and more, but it also plays a key role in the pathogenesis of these diseases. 

IL-17 plays a role in tumorigenesis and transplant rejection.

Interleukin 2

There are several types of interleukins, numbered from IL-1 to IL-38, with each type having its own specific functions.

For example, IL-1 is involved in initiating inflammation, while IL-2 plays a role in stimulating the proliferation of T cells, a type of immune cell.

Other interleukins, such as IL-6 and IL-10, have pro-inflammatory or anti-inflammatory effects, respectively

interleukins contribute to our understanding of the immune system and its involvement in various diseases.

Some therapeutic interventions, such as targeted drugs or immunotherapies, aim to modulate or block specific interleukins to treat conditions like autoimmune diseases, cancer, and inflammatory disorders.


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