Fig. 1.

Cell extrinsic recognition of pathogens. Bacteria detected by dendritic cells through Toll like receptors (TLRs) are internalized into phagosome where bacterial antigens are processed for presentation on MHC class II. Bacterial antigens (red) and pathogen-associated molecular patterns (PAMPs) (blue) are present in the same phagosome, which indicates to the decdritic cell their common origin. TLR-mediated recognition of bacterial PAMPs promotes the selection of bacterial antigens for optimal presentation on MHC class II. TLR signaling also leads to the induction of costimulatory molecules and cytokines necessary for activation and differentiation of T lymphocytes.