The interleukin-2 receptor alpha chain (also called Tac antigen, P55, and mainly CD25) is a protein involved in the assembly of the high-affinity interleukin-2 receptor, consisting of alpha (IL2RA), beta (IL2RB) and the common gamma chain (IL2RG). This receptor interacts with interleukin-2, a pleiotropic cytokine which plays an important role in immune homeostasis. Quantification of soluble IL2RA is a useful and simple tool for clinicians to assess immune function in vivo as part of the investigation, management, and prognosis of a broad spectrum of diseases, such as sarcoidosis, Multiple Sclerosis, primary biliary cirrhosis, Chronic immune activation in common variable immunodeficiency (CVID), chronic liver diseases and many more.