CD38, also known as cyclic ADP ribose hydrolase, is a multifunctional type 2 transmembrane glycoprotein found on the surface of various immune cells, including T cells, B cells, granulocytes, and NK cells. It consists of an intracellular domain, a transmembrane domain, and an extracellular domain with enzymatic activity. CD38 acts as both a receptor and an enzyme, regulating cell adhesion and participating in signal transduction. It metabolizes extracellular NAD+ to produce ADPR and cyclic ADPR, which are crucial for controlling intracellular calcium fluxes and maintaining nucleotide homeostasis. It is highly expressed in plasma cells and some solid tumors, and its expression is regulated by factors like IFN type I and II. CD38 plays a significant role in immune responses, including phagocytosis and antigen presentation. It is also involved in various physiological and pathological processes such as inflammation, tumorigenesis, and aging. In clinical practice, CD38 serves as a cell activation marker and a diagnostic tool for certain cancers. Additionally, anti-CD38 monoclonal antibodies are used in the treatment of multiple myeloma and are being investigated for their potential in treating other diseases.