Fig. 3.

Hydrophobic spacer lipids can fill the empty cavity of CD1 isoforms as shown in the structure CD1c with mannosyl-phosphomycoketide (MPM) and CD1d with short-chain α-galactosylceramide (PBS-25). When higher affinity lipids are present in permissive environments, ligands and spacers must both be displaced during antigen exchange reactions.  To illustrate this, CD1 is rendered with antigenic lipids in blue with hydrophobic spacer lipids in red, with schematic [figures were generated from RCSB protein data bank files 3OV6 (CD1c-MPM), 1Z5L (CD1d-PBS-25), 3T8X (CD1b-sulfoglycolipid), and 1UQS (CD1b-GMM)]