Question

Please describe the comparison between the class Ⅰ and class Ⅱ HLA molecules.

Answer 1

Comparison of class I and class II Human leukocyte antigen (HLA);

Class I HLA molecules encoded on glycoproteins expressed on the surface of nearly all nucleated cells; the major function of the class I gene product is presentation of peptide antigens to TC cells.
Class II HLA molecules encoded on glycoproteins expressed primarily on APCs, where they present processed antigenic peptides to TH cells.

Major difference between HLA Class I and HLA Class II (including their antigen processing and presentation pathway) is summarized in this table:

	HLA Class I	HLA Class II
Structure	HLA class I molecules consist of one membrane-spanning α chain (heavy chain) produced by MHC genes, and one β chain (light chain or β2-microglobulin) produced by the β2-microglobulin gene.	HLA class II molecules consist of two membrane-spanning chains, α and β, of similar size and both produced by MHC genes.
Types of APCs	HLA I glycoproteins are present in all nucleated cells.	HLA II glycoproteins are only present on specialised antigen-presenting cells (APCs), including macrophages that engulf foreign particles such as bacteria, dendritic cells that present antigen to T cells, and B cells that produce antibodies.
Nature of Antigen Presentation	HLA class I glycoproteins present endogenous antigens that originate from the cytoplasm.	HLA II proteins present exogenous antigens that originate extracellularly from foreign bodies such as bacteria.
Size of peptide	HLA Class I present 8-10 amino acid peptides	HLA Class II presents 14-18 amino acid peptides.
Responsive T Cells	Present antigen to cytotoxic T cell lymphocytes (CD8+ T Cells);	Present antigen to helper T cell lymphocytes; (CD4+ T cells).
Co-receptor responsible	Binds with CD8 coreceptors molecules on cytotoxic T cells	Binds with CD4 co-receptors molecules on helper T cells
Sources of Protein Antigens	Cytosolic proteins (mostly synthesized in the cell, may enter cytosol from phagosomes)	Endosomal/lysosomal proteins (mostly internalized from extracellular environment)
Enzymes Responsible for peptide generation	Cytosolic proteasome	Endosomal and lysosomal proteases (e.g., cathepsins)
Site of peptide loading of HLA	Endoplasmic reticulum	Specialized vesicular compartment
Molecules involved in transport of peptides and loading of HLA molecules	Chaperones, TAP in ER	Chaperones in ER; invariant chain in ER, Golgi and HLA Class II compartment/Class II vesicle; DM
End Result	Presentation of foreign-intracellular antigens or altered self-antigens; targets cell for destruction	Presentation of foreign extracellular antigens; induces antibody production, and attracts immune cells to area of infection