ICOS (The inducible T cell co-stimulator) belongs to the CD28 family of co-stimulatory molecules, and it is selectively expressed by activated T-cells, while its ligand is constitutively present on the surface of a variety of cells including dendritic cells, macrophages, B-cells but also on endothelial cells, lung epithelial cells and fibroblasts. ICOS has been implicated in various immune outcomes, including the induction and regulation of Th1, Th2, and Th17 immunity. The triggering of ICOS on T-cells by ICOSL has been shown to modulate lymphocyte cytokine secretion pattern and, in some conditions, to favour regulatory T- cell (Treg) differentiation. ICOS cooperate in a nonredundant manner to maximize and sustain Th cell immune responses. In addition, ICOS/ICOSL Interaction plays an important role in the development and differentiation of Follicular T-helper cells (Tfh) in the germinal centres of lymphatic nodes. Mouse and human ICOS share approximately 72% amino acid identity.