HLA-D is the cell surface component which plays this role.
Finally, the astrocytes (star cells) of the brain itself can, when excited, express HLA-D to present antigen, in this case MBP, to the T-cells.
The HLA-D molecule is a protein made up of two different peptides, (peptides are chains of amino acids, the basic building blocks of all proteins) called alpha and beta.
Immunologists, who study these things, like to compare the exact fit of the MBP with HLA-D, and of the MBP-HLA-D complex with the TCR, with the perfect fit of keys in the locks for which they are designed.
These are the genes that correspond to the alpha and beta chains of HLA-D and the alpha and beta chains of the TCR.
This approach showed an association between MS and an HLA-D marker known as DR2 (and less frequently DR4 and DR6).