Chapter 3 Review Questions

When antigens enter through the skin, in what organs are they concentrated? What cell type(s) play important roles in this process of antigen capture?

Antigens that enter through the skin are captured by dendritic cells that reside in the epidermis and dermis and transported to skin draining lymph nodes, where the antigens are concentrated and displayed to lymphocytes.

Which subsets of T cells recognize antigens presented by class I and class II MHC molecules? What molecules on T cells contribute to their specificity for either class I or class II MHC-associated peptide antigens?

CD4+ helper T cells recognize antigens displayed by class II molecules, and CD8+ cytotoxic T lymphocytes recognize MHC class I-peptide complexes. The CD4 co-receptor of helper T cells can bind to MHC class II molecules, and the CD8 co-receptor of cytotoxic T cells binds to MHC class I molecules.

Describe the sequence of events by which class I and class II MHC molecules acquire antigens for display.

MHC class II α and β chains are produced in the endoplasmic reticulum, where they assemble with each other and with an invariant chain that occludes the antigen binding cleft. The MHC class II-invariant chain complex is transported to a late endosomal/lysosomal compartment where the invariant chain is degraded, leaving a peptide called CLIP in the cleft. Proteins internalized by the endocytic pathway are degraded in late endosomes and lysosomes into peptides. Specific peptides displace CLIP and bind tightly to the cleft of the MHC class II molecule, which is then transported to the cell surface.

What are MHC molecules? What are human MHC molecules called? How were MHC molecules discovered, and what is their function?

Major histocompatibility complex (MHC) molecules are cell surface proteins that present antigenic peptides to T cells. Human MHC proteins are called HLA molecules. They were initially discovered as products of polymorphic genes that mediate transplant rejection. Their physiologic function is antigen presentation.

What are the differences between the antigens that are displayed by class I and class II MHC molecules?

Proteins that are produced in or enter the cytosol are presented by MHC class I molecules. Proteins internalized into vesicles by endocytosis are presented by MHC class II molecules.