CD1d is a non-polymorphic, MHC class I-like glycoprotein that plays a crucial role in presenting lipid antigens to invariant natural killer T （iNKT） cells. It is widely expressed in both hematopoietic and non-hematopoietic cells, including professional antigen-presenting cells （APCs）, thymocytes, hepatic stellate cells, hepatocytes, adipocytes, and intestinal epithelial cells. Structurally, CD1d consists of a heavy chain with three extracellular domains （α1, α2, and α3） that interact non-covalently with beta-2-microglobulin （B2M）. The antigen-binding groove of CD1d, formed by two anti-parallel α helices and one anti-parallel β helix, is deeper than that of typical MHC class I molecules and is specialized for binding lipid antigens. CD1d presents a wide range of lipid antigens, including diacylglycerols, phospholipids, glycosphingolipids, ether lipids, and lipopeptides, to CD1d-restricted T cells. This interaction is critical for activating iNKT cells, which have potent immunoregulatory functions and contribute to immune responses against cancer, infections, and autoimmune diseases.