Activation of dendritic cells by toll-like receptors and C-type lectins.

As sentinels of the immune system, dendritic cells (DC) scan their environment for the presence of pathogens. DC sense pathogens either directly or indirectly via endogenous factors such as cytokines and chemokines, which are produced by other cell types in response to infection. Although indirect signals in form of endogenous factors alert DC, direct activation of DC by pathogen-associated molecular patterns (PAMP) is crucial for the induction of primary T cell responses. Direct recognition of PAMP is mediated by pattern recognition receptors (PRR) such as Toll-like receptors (TLR) and C-type lectin receptors (CLR). The molecular patterns that are recognized by these receptors are indispensable for the life cycle of the pathogens, and their structure or cellular localization is different from that of the host. TLR detect cell-wall components of bacteria, fungi, and protozoa at the cell surface or bacterial and viral nucleic acid structures in a specialized endosomal compartment, while CLR that are involved in pattern recognition bind to carbohydrate structures associated with pathogens.