The difference between MHC I and MHCII ?

Class I MHC: Expressed on all nucleated cells Consists of a single α chain tethered to the cell membrane non-covalently associated with a β-microglobulin molecule. Polymorphic regions lie in α1 and α2 domains which form a cleft big enough to accommodate peptides 8-11 amino acids long. α3 domain is invariant and contains the binding site for the CD8 co-receptor Class II MHC: Expressed on dendritic cells, B cells, macrophages, thymic epithelial cells, some endothelial cells. Consists of α and β chains. Polymorphic residues lie in α1 and β1 domains which form a cleft which can accommodate peptides 10-30 amino acids long. β2 domain is CD4 co-receptor binding site.