<P> Non-professional antigen presenting cells include all nucleated cell types in the body . They use an MHC class I molecule coupled to beta - 2 microglobulin to display endogenous peptides on the cell membrane . These peptides originate within the cell itself, in contrast to the exogenous antigen displayed by professional APCs using MHC class II molecules . Cytotoxic T cells are able to interact with endogenous antigen presented using an MHC class I molecule . Non-professional APCs do not typically express MHC class II molecules . However, it has been observed that antigen presentation to CD4+ cells via MHC class II is not restricted to the classically professional APCs . Other leukocytes, including granulocytes such as mast cells and neutrophils, can be induced to do so, as can endothelial and epithelial cells under certain circumstances . Even so, there is little evidence that these atypical APCs are able to activate naive CD4+ T cells . </P> <P> After dendritic cells have phagocytosed pathogens, they usually migrate to the vast network of lymph vessels and are carried by lymph flow to the draining lymph nodes . Each lymph node is a collection point where APCs can interact with T cells . During the migration, DCs undergo a process of maturation: they lose most of their ability to further engulf pathogens and they mature by changing surface expression of MHC and co-stimulatory molecules, as well as increased production of cytokines . The internalized antigen is digested into smaller peptides containing epitopes, which are then presented to T cells by the MHC . </P> <P> B cells reside in the lymph node . Once their B cell receptor binds to an antigen, they can interact with activated helper T cells, as described above . </P> <P> A dendritic cell that interacts with an already - activated helper T cell can become licensed . This occurs through the interaction of co-stimulatory molecules including B7 and CD40 on the dendritic cell, with CD28 and CD40 ligand on the T cell . Only licensed dendritic cells are able to activate cytotoxic T cells . T cell licensing of dendritic cells is key for activation of cytotoxic T cells for many pathogens, although the extent to which T cell help is needed may vary . </P>

Apc play all of the following role in the immune response except