CD1 molecules belong to non-polymorphic MHC class I-likeproteins and present lipid antigens to T cells. Five differentCD1 genes (CD1a-e) have been identified and classified intotwo groups. Group 1 include CD1a-c and present pathogeniclipid antigens to αβ T cells reminiscence of peptide antigenpresentation by MHC-I molecules. CD1d is the only memberof Group 2 and presents foreign and self lipid antigens to aspecialized subset of αβ T cells, NKT cells. NKT cells areinvolved in diverse immune responses through prompt andmassive production of cytokines. CD1d-dependent NKT cellsare categorized upon the usage of their T cell receptors. Amajor subtype of NKT cells (type I) is invariant NKT cellswhich utilize invariant Vα14-Jα18 TCR alpha chain in mouse. The remaining NKT cells (type II) utilize diverse TCR alphachains. Engineered CD1d molecules with modified intracellulartrafficking produce either type I or type II NKT celldefectssuggesting the lipid antigens for each subtypes of NKTcells are processed/generated in different intracellular compartments. Since the usage of TCR by a T cell is the result ofantigen-driven selection, the intracellular metabolic pathwaysof lipid antigen are a key in forming the functional NKT cellrepertoire. [BMB Reports 2014; 47(5): 241-248]