IL5

Interleukin-5 (IL-5) is a cytokine produced primarily by T helper type 2 (Th2) cells and other immune cells. It plays a central role in the growth, differentiation, recruitment, activation, and survival of eosinophils, a type of white blood cell involved in defense against parasites and in allergic inflammation. IL-5 acts as a key driver of eosinophilopoiesis in the bone marrow and modulates eosinophil function at sites of inflammation.

Genetics and structure: The IL5 gene is located on chromosome 5q31-33 in humans, in a cytokine gene cluster that includes several other interleukins. The mature IL-5 protein is a secreted glycoprotein of about 133 amino acids and forms a functional homodimer. It belongs to the four-helix bundle family of cytokines and is produced by activated T cells, mast cells, and eosinophils.

Receptor and signaling: IL-5 signals through the IL-5 receptor, a heterodimer composed of the IL-5 receptor alpha chain (IL-5Rα) and the common beta chain (βc). Engagement of the receptor activates downstream signaling pathways, including JAK/STAT, PI3K, and MAPK, which promote eosinophil progenitor proliferation, differentiation, and survival.

Biological function: Beyond stimulating eosinophil production, IL-5 enhances eosinophil survival and activation, contributing to eosinophilic inflammation in tissues such as the airways during allergic responses and asthma, as well as during certain helminth infections. IL-5 operates in concert with other cytokines, notably IL-3 and GM-CSF, to regulate eosinophil biology.

Clinical relevance: Abnormal or elevated IL-5 activity is linked to eosinophilic diseases and asthma. Therapeutic strategies have targeted IL-5 or its receptor to reduce eosinophil counts and disease exacerbations, including monoclonal antibodies such as mepolizumab and reslizumab (targeting IL-5) and benralizumab (targeting IL-5Rα).