The human immune system is a complex network of cells and organs that work together to protect the body from foreign invaders like bacteria, viruses, and other pathogens. This communication often involves a specialized group of signaling proteins that act as messengers, orchestrating the body’s protective actions.
What Interleukins Are
Interleukins are signaling proteins, also known as cytokines, that are produced by white blood cells (leukocytes) and other body cells. The term “interleukin” itself reflects their primary function: “inter-” meaning “between,” and “-leukin” referring to their origin and action on leukocytes. While initially thought to be exclusively produced by leukocytes, many other cell types in the body also produce interleukins. More than 50 interleukins and related proteins have been identified, each playing a specific role in regulating immune responses. They are not stored within cells but are instead rapidly secreted in response to various stimuli, such as infections or inflammation.
How Interleukins Work
Once an interleukin is produced and secreted by a cell, it travels through the body to reach its intended target. Upon arrival, the interleukin binds to specific receptor molecules located on the surface of the target cell. This binding event is often compared to a lock-and-key mechanism, where only the correct interleukin can fit into its corresponding receptor.
The binding of an interleukin to its receptor triggers a cascade of signals inside the target cell. This intracellular signaling pathway ultimately alters the cell’s behavior, leading to a specific response. Different interleukins bind to different receptors and activate distinct signaling pathways, resulting in diverse effects on various cell types. For example, the binding of Interleukin-1 (IL-1) to its receptor can initiate a signaling cascade that activates transcription factors like NF-κB, which are important for inflammatory responses.
Interleukins in Immunity
Interleukins play diverse and complex roles in orchestrating the body’s immune responses. They are involved in activating and differentiating immune cells, as well as influencing their proliferation, maturation, migration, and adhesion. These proteins can exhibit both pro-inflammatory and anti-inflammatory properties, finely tuning the immune system’s response.
Some interleukins, such as Interleukin-1 (IL-1) and Interleukin-6 (IL-6), are known for promoting inflammation. IL-1, for instance, can induce fever and stimulate the production of acute-phase proteins, which are part of the body’s immediate response to infection or injury. IL-6 also plays a role in chronic inflammatory diseases and can act as a pro-inflammatory cytokine.
Other interleukins, like Interleukin-2 (IL-2), are crucial for regulating immune cell growth and differentiation, particularly of T cells. Interleukins also contribute to attracting immune cells to sites of infection or injury, a process known as chemotaxis. While chemokines are primarily responsible for guiding immune cells, some interleukins can also act as chemoattractants for certain immune cells.
Conversely, some interleukins suppress immune responses to prevent overactivity and autoimmunity. Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine that helps to mitigate excessive inflammation and tissue damage. Interleukin-35 (IL-35) is another anti-inflammatory cytokine that can reduce the progression of autoimmune diseases by contributing to immune suppression. The balance between these pro-inflammatory and anti-inflammatory interleukins is important for maintaining immune homeostasis.
Interleukins and Health
Imbalances in interleukin activity can contribute to various health conditions. In infectious diseases, interleukins are signaling molecules that regulate immune responses. Pro-inflammatory interleukins help activate and recruit immune cells to fight off pathogens, while anti-inflammatory interleukins help suppress excessive inflammation and promote tissue repair.
Dysregulation of interleukins can lead to autoimmune diseases, where the immune system attacks the body’s own tissues. For example, elevated levels of pro-inflammatory interleukins like IL-1, IL-6, and IL-17 are associated with conditions such as rheumatoid arthritis and Crohn’s disease. In rheumatoid arthritis, IL-1β plays a role in disease progression, and IL-17 is found at elevated levels in affected joints. Similarly, increased levels of IL-6 are observed in patients with inflammatory bowel disease, including Crohn’s disease.
Interleukins also have a complex role in cancer, sometimes promoting and sometimes inhibiting tumor growth. Some interleukins, like IL-2, IL-12, and IL-15, can activate immune cells to target and destroy cancer cells. However, other interleukins, such as IL-6 and IL-10, can contribute to tumor progression by promoting cell growth, suppressing anti-tumor immune responses, or creating an environment that helps cancer cells evade the immune system.
Understanding these roles has led to new therapeutic applications, where interleukin modulators are used to treat inflammatory diseases and certain cancers. For instance, laboratory-made versions of IL-2 are approved to treat advanced kidney cancer and metastatic melanoma. Inhibitors targeting specific interleukins are also used in autoimmune conditions to reduce inflammation and disease progression.