MHC and Antigen Presentation

A 32-year-old woman from Delhi presents with recurrent severe infections caused by intracellular pathogens including Mycobacterium tuberculosis and Listeria monocytogenes. Immunological workup reveals normal CD4+ and CD8+ T cell counts, but flow cytometry shows absent HLA-DR expression on antigen-presenting cells. Her macrophages fail to present processed antigens to helper T cells in vitro. Which of the following genetic defects best explains her immunodeficiency?

A 28-year-old man from Mumbai presents with recurrent pyogenic infections (Staphylococcus aureus, Streptococcus pneumoniae) and chronic diarrhea since infancy. Immunological investigation shows normal T and B cell counts. Flow cytometry reveals markedly reduced HLA-A, HLA-B, and HLA-C expression on all nucleated cells, but HLA-DR expression is preserved. In vitro cytotoxic T lymphocyte (CTL) responses to viral antigens are severely impaired. Which of the following best explains the molecular basis of his immunodeficiency?

Which of the following molecules is responsible for the stabilization of peptide-MHC class I complexes on the cell surface?

Which MHC class II molecule is responsible for presenting exogenous antigens to CD4+ T helper cells after processing in the endosomal-lysosomal compartment?