Which statement best describes the trigger for antigen presentation by class II MHC proteins?

• A. A third signal that directly activates T cells.
• B. Phagocytic antigen-presenting cells engulf extracellular pathogens.
• C. Endogenous peptides from intracellular pathogens are presented by class II MHC.
• D. All nucleated cells express class II MHC without restriction.

Answer: (B)

Class II MHC molecules present peptides from proteins that come from outside the cell, to CD4+ T helper cells. The key trigger is uptake of extracellular antigens by professional antigen-presenting cells, such as dendritic cells, macrophages, and B cells. After phagocytosis or endocytosis, the ingested proteins are processed in endosomal compartments and the resulting peptides are loaded onto MHC II molecules for transport to the cell surface. Endogenous (intracellular) pathogens yield peptides that are presented by class I MHC, not class II, and expression of class II MHC is restricted to professional APCs rather than all nucleated cells. A co-stimulatory signal helps activate T cells after recognition, but it is not what initiates class II antigen presentation.