ABSTRACT
The presence of an invading microorganism is first recognized by tissue macrophages at the site of infection. Pathogens carry characteristic chemical structures on their surface and in their nucleic acids that are not present in human cells. These pathogen-associated molecular patterns (PAMPs) are recognized by invariant pattern recognition receptors (PRRs) on macrophages, and also on neutrophils, which take up the pathogen and destroy it. PPRs are also present on dendritic cells, whose function is to present antigen to and activate naive T cells; dendritic cells form the link between the innate and the adaptive immune response. Among the PRRs are the Toll-like receptors (TLRs), which are present on the cell surface and in the membranes of endosomes, and which enable the cells of the innate immune system to detect and respond to a wide variety of pathogens.
