Tim-1 is a transmembrane protein also known as T cell immunoglobulin and mucin domain containing protein-1 and hepatitis virus cellular receptor 1. It is developmentally expressed at high levels in the blastocyst. Tim-1 is expressed on activated CD4+ lymphocytes especially on Th2 cells and has been implicated to play a critical role in the development of atopic disease and other Th2-biased immune responses. Tim-1 is hepatitis A virus receptor in humans. Tim-4 is the endogenous ligand of Tim-1. The interaction of Tim-1 and Tim-4 is involved in costimulation of T cell proliferation. Tim-1 is an endogenous ligand for LMIR5/CD300b.