CD278, also known as ICOS (inducible T-cell co-stimulator), is a member of the CD28 superfamily and is expressed on activated T cells, including regulatory T cells (Tregs) and follicular helper T cells (Tfh). It functions as a homodimer and plays a crucial role in T cell activation, proliferation, and differentiation. ICOS binds to its ligand ICOSL (CD275), which is expressed on antigen-presenting cells (APCs) such as dendritic cells, B cells, and macrophages. This interaction enhances T cell-mediated immune responses, promotes cytokine production (e.g., IL-4, IL-13), and supports the survival and function of Tfh cells, which are critical for B cell antibody production. ICOS signaling is also involved in Th2 and Th17 cell differentiation and has been implicated in autoimmune and inflammatory diseases. In cancer immunotherapy, ICOS agonist antibodies have shown potential for enhancing anti-tumor immune responses when combined with other immunotherapies.