Tim-1 is a transmembrane glycoprotein expressed on activated CD4+ lymphocytes especially on Th2 cells. It 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. Recently, Tim-1 has been reported to be an endogenous ligand for LMIR5 (Leukocyte mono-immunoglobulin (Ig)-like receptor 5), and the Tim-1-LMIR5 interaction plays a physiological role in immune regulation by myeloid cells.