This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Antigen presented along with HLA-ll stimulates \nSelect and explain. \nA. CD8 cells, B. CD4cells, C. CD2 cells, D. CD19 cells', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ref Robbins 9/e p195 Class II MHC molecules are encoded by genes in the HLA-D region, which contains at least three subregions: DP, DQ, and DR. Class II MHC molecules are heterodi- mers of noncovalently linked polymorphic a and b sub- units (Fig. 4-3). The extracellular poion of the class II MHC heterodimer contains a cleft for the binding of antigenic peptides and a region that binds CD4. Class II MHC expression is restricted to a few types of cells, mainly APCs (notably, dendritic cells ), macro- phages, and B cells. In general, class II MHC molecules bind to peptides derived from proteins synthesized outside the cell (e.g., those derived from extracellular bacteria) and ingested into the cell. This propey allows CD4+ T cells to recognize the presence of extracellular pathogens and to orchestrate a protective response.', 'role': 'assistant'}]