Table 1.

Contrasting features of MHC Class I, MHC Class II, and CD1 antigen presentation systems

Feature	MHC Class I	MHC Class II	CD1
Recognized ligands	Peptides	Peptides	Lipids
Ligand processing	Peptides derived from proteasomal degradation of proteins	Peptides derived from lysosomal degradation of proteins	Usually not required
Ligand origin	Endogenous	Exogenous	Exogenous or endogenous
Ligand size and shape	8–9 amino acids long peptides	14–20 amino acids long peptides	Different lipid types of varying shapes and sizes
Binding site structure	Closed-ended groove	Open-ended groove	Larger clefts of varying shapes and sizes, from dead-end single-pass tunnels to riddled maze-like structures
Expression	Most cell types	Antigen-presenting cells	CD1a-c mostly in antigen-presenting cells, CD1d more widely
Degree of polymorphism	Highly polymorphic	Highly polymorphic	Non-polymorphic
Degree of polygenicity	Highly polygenic	Highly polygenic	Non-polygenic (in humans)
Responding T cells	CD8+	CD4+	αβ, γδ, CD4+, CD8+ or double-negative (CD4−CD8−), iNKT, NKT type II