CD1D is a non-classical MHC class I-like glycoprotein specializing in lipid antigen presentation to T cells, particularly natural killer T (NKT) cells. Unlike classical MHC molecules, it presents lipid and glycolipid antigens via a unique binding groove. Expressed on antigen-presenting cells, CD1D bridges innate and adaptive immunity by activating NKT cells to secrete cytokines, influencing responses to infections, autoimmunity, and cancer. Genetic variants are associated with diseases like type 1 diabetes.