<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> <P> In MHC class I and class II molecules, only certain epitopes of an internalized peptide can be presented . These epitopes are termed immunodominant . </P> <P> APCs naturally have a role in fighting tumors, via stimulation of B and cytotoxic T cells to respectively produce antibodies against tumor - related antigen and kill malignant cells . Dendritic cells, presenting tumor - specific antigen to T cells, are key to this process . Cancer therapies have included treating the patient with increased numbers of dendritic cells or cancer - specific T cells . However, newer therapies have turned to genetically engineered artificial antigen - presenting cells designed to prime the immune system to attack malignant cells . Some artificial APCs are derived from human cells; others are acellular, containing MHC proteins, co-stimulatory molecules and the necessary peptides . </P> <P> The APC activator IMP321 is being tested in clinical trials to accelerate the immune reaction to eliminate metastatic breast cancer or melanoma . </P>

These phagocytic cells present fragments to t cells for activation