Human Thymic Stromal Lymphopoietin (TSLP) is a cytokine belonging to the IL-2 cytokine family, which also includes IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21, molecules that regulate lymphocyte development and immune responses. In humans, TSLP is produced primarily by epithelial cells, stromal cells, keratinocytes, and fibroblasts at barrier tissues such as the skin, respiratory tract, and gastrointestinal tract, particularly in response to infection, allergens, tissue damage, or inflammatory stimuli. TSLP signals through a heterodimeric receptor composed of the TSLP receptor (TSLPR/CRLF2) and the IL-7 receptor alpha chain (IL-7Rα), activating intracellular signaling pathways including JAK/STAT, especially STAT5, which regulate immune cell differentiation and cytokine production. In human immunity, TSLP plays a key role in initiating type 2 immune responses by activating dendritic cells that promote differentiation of T helper 2 (Th2) cells, leading to production of cytokines such as IL-4, IL-5, and IL-13. TSLP is particularly important in allergic inflammation and barrier immunity, and elevated TSLP expression has been strongly associated with allergic diseases including asthma, atopic dermatitis, allergic rhinitis, and eosinophilic esophagitis. Because of its central role in type 2 inflammatory pathways, human TSLP has become an important therapeutic target, with TSLP-blocking biologics (such as tezepelumab) developed for treatment of severe asthma and other allergic disorders.