Background：CD161 is a type II transmembrane glycoprotein, also known as NKR-P1A, that is expressed as a 40-44 kD homodimer. It is a member of the C-type lectin superfamily. CD161 is expressed on a majority of NK cells, NKT cells, and subsets of peripheral T cells and CD3+ thymocytes. It has been reported that Th17 cells are a subpopulation of CD4+CD161+CCR6+ cells. While the biological function of CD161 is not clear, it has been suggested to serve either as a stimulatory receptor or to inhibit NK cell-mediated cytotoxicity and cytokine production. LLT-1 （lectin-like transcript-1, also named as osteoclast inhibitory lectin or CLEC2D） is the ligand of CD161.
