Blood & Laboratory Values

Interleukins – Function & Diseases


Interleukins form a subgroup of cytokines , cellular messengers that control the immune system . The interleukins are short-chain peptide hormones with 75 to 125 amino acids . They mainly control the local use of leukocytes at sites of inflammation, but they can also have a systemic effect, such as the triggering of fever .

What are interleukins?

Interleukins ( IL ) are short-chain peptide hormones with 75 to 125 amino acids. They form one of several subclasses of cytokines that control the immune system. As a messenger substance, interleukins have a similar range of uses as interferons, which also form a subclass of cytokines.

However, interleukins are particularly specialized in controlling leukocytes. Some interleukins also show systemic effects, for example by being able to induce fever, while interferons are more specialized in fighting off viruses and have antitumor abilities. Unlike neurotransmitters , interleukins and interferons specialize in communicating with cells in the immune system and with tissue cells. Their main effect usually takes place locally in the tissue.

In order to communicate with cells of the immune system or with tissue cells, interleukins do not have to penetrate into the cells, they simply dock onto specific receptors of the cells, which is sufficient to cause the immune cells to proliferate, differentiate and become active.

Function, effect & tasks

Each of the more than 40 different interleukins performs a specific task. Overall, interleukins control the use of leukocytes , but also partly the use of T helper cells, monocytes and macrophages as well as other immune cells. 

The basic tasks consist of stimulating cells of the immune system to mature, grow and divide, i.e. to multiply, if necessary. This also includes the opposite process, the reversal of certain immune reactions. Interleukin-1 has the ability to produce fever under certain conditions. IL-1 is therefore one of the so-called pyrogens together with IL-6 and tumor necrosis factor . The IL-2 specializes in the stimulation, proliferation and differentiation of T helper cells, B cells and natural killer cells. The most important task of the IL-3 consists in the emission of stimuli, which transform certain pluripotent stem cells into erythrocytes , granulocytesor other cells of the immune system.

IL-4 also has the ability to transmit proliferation and differentiation stimuli to T cells, but at the same time it also has an inhibitory effect on macrophage activity. IL-4 thus also has an anti-inflammatory effect. In addition to all cell types that belong to the immune system, target cells of certain interleukins can also be stromal cells or fibroblasts , as with IL-17. In order to modulate the inflammatory processes in the skin , interleukin-20 probably directly controls the immune response of the keratinocytes in the top layer of the skin.

A few interleukins such as IL-28 and IL-29 recognize virus-infected cell lines. IL-24 is probably the only interleukin that can recognize tumor cells and has a tumor-inhibiting effect by inhibiting growth and inducing cell apoptosis, the self-induced cell death.

Formation, Occurrence, Properties & Optimal Values

Most interleukins are released by cells with immunological relevance mainly into the intercellular area, where they can dock to the secreting cell itself or to other cells of the immune system. Only in a few exceptional cases do specialized interleukins occupy receptors on cells that do not belong to the immune system.

An exception is, for example, IL-33, which is released in the lungs and skin and can dock to receptors of the IL-1 family. As with IL-4, IL-5 and IL-13, the target cells are mostly T cells and sometimes also eosinophils and mast cells. In principle, the communication between the cells is in the foreground with interleukins. It is usually a small-scale, locally effective communication, with systemic effects also being achieved in exceptional cases. Some interleukins resemble growth factors because their influence on T cells, monocytes and lymphocytes is comparable to that of growth factors.

Due to the high dynamics resulting from the changing demands on the immune system, it makes no sense to state a reference value or an optimal value for their occurrence in the body. However, problems can arise due to reduced or excessive secretion, as is observed in allergic reactions, for example.

Diseases & Disorders

The very complex interaction of the individual components of the immune system causes a large number of possible disorders, weakening of the immune response or an excessive reaction to certain challenges, which can lead to mild to severe symptoms of the disease. 

In some cases, however, the secretion of the cytokines is not disturbed, but the problem lies with disturbed receptors to which interleukins and other cytokines cannot dock. The immune response to tissue inflammation is dominated by IL-1. As an inflammatory signaling substance, its activity can be abnormally increased so that not only dead body tissue is phagocytosed and transported away, but healthy cells are also attacked and diseases such as rheumatism and arthrosis in the joints are caused. In these cases, an antagonist to IL-1 can help, which suppresses the immune response through IL-1.

Antagonists to IL-1 can also be used in other autoimmune diseases such as Crohn’s disease , MS and psoriasis . Because interleukins are made up of relatively short-chain proteins or polypeptides, most of them can also cross the blood-brain barrier . In some cases, transport is performed by specialized astrocytes . Although no direct specificity of the individual interleukins with regard to schizophrenia and depressionexists, clear connections can be found, for example, between hypersecretion of IL-2 in schizophrenia and of IL-6 in depression. Interleukins and other cytokines exert strong influence on neurotransmitters such as dopamine , serotonin , adrenaline , noradrenaline and others.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.