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ID: 257360 • Letter: Y

Question

You received no credit for this question in the previous attempt. View previous Which of the following statements is correct about antigen presentation by class I and class II MHC molecules? Multiple Choice Class I MHC molecules bind to fragments that initially come from antigens outside the cell, thus they undergo exogenous antigen processing. An antigen-presenting cell (APC), such as a macrophage, dendritic cell, or B cell, takes in the MHC molecule by receptor-mediated endocytosis or phagocytosis and produces MHC fragments by digestion in the phagolysosome. The class I MHC molecule is presented on the surface together with its endogenous antigen, enabling the host cell to present the antigen to a subset of T cells known as cytotoxic T lymphocytes. Class II MHC molecules bind to fragments that initially come from antigens outside the cell, thus they undergo endogenous antigen processing.

Explanation / Answer

3. Class I MHC molecule is presented on the surface together with its endogenous antigen enabling the host cell to present the antigen to a subset of T cells known as cytotoxic T lymphocytes.

For understanding : enclosed the differences between Class I and II MHC molecules.

MHC Class I

MHC Class II

Structure

MHC class I molecules consist of one membrane-spanning ? chain (heavy chain) produced by MHC genes, and one ? chain (light chain or ?2-microglobulin) produced by the ?2-microglobulin gene.

MHC class II molecules consist of two membrane-spanning chains, ? and ?, of similar size and both produced by MHC genes.

Types of APCs

MHC I glycoproteins are present in all nucleated cells.

MHC 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

MHC class I glycoproteins present endogenous antigens that originate from the cytoplasm.

MHC II proteins present exogenous antigens that originate extracellularly from foreign bodies such as bacteria.

Size of peptide

MHC Class I present 8-10 amino acid peptides

MHC 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 MHC

Endoplasmic reticulum

Specialized vesicular compartment

Molecules involved in transport of peptides and loading of MHC molecules

Chaperones, TAP in ER

Chaperones in ER; invariant chain in ER, Golgi and MHC 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