 This process is essential when APCs are not directly infected, triggering local antiviral and anti-tumor immune responses immediately without trafficking the APCs in the local lymph nodes.Antigens from the extracellular space and sometimes also endogenous ones, are enclosed into endocytic vesicles and presented on the cell surface by MHC-II molecules to the helper T cells expressing CD4 molecule.Only APCs such as dendritic cells, B cells or macrophages express MHC-II molecules on their surface in substantial quantity, so expression of MHC-II molecules is more cell-specific than MHC-I.