ICOS also known as CD278 is a member of the CD28 family that regulates T cell activity and immune responses. ICOS plays a crucial role in T cell activation and regulation, functioning as a key player in the immune response. ICOS is primarily expressed on activated T cells and is essential for enhancing T cell responses, particularly in the context of T follicular helper cells, which are vital for effective antibody production. The interaction of ICOS with B7h leads to the upregulation of various cytokines, including IL-10, which is important for maintaining immune homeostasis and preventing excessive inflammation. ICOS does not induce IL-2 upregulation, distinguishing ICOS function from CD28, which is critical for T cell proliferation.