Pattern recognition receptors

The term "pattern recognition receptor" (PRR) describes a diverse set of receptors that recognize and differentiate between incoming microbial challenges and damage to self. These receptors initiate and direct all aspects of the immune response; some trigger soluble cascades, others bind to pathogens, and many initiate complex changes in cellular behavior. The consequences of PRR engagement include triggering inflammation, antipathogen cascades, as well as activating and directing cells of the immune responses. Many of these receptors are highly conserved in the animal kingdom (and PRR systems are also present in plants), it is therefore unsurprising that so many can be detected in birds. The avian PRRs survey all areas of the body, some are secreted onto mucosal surfaces, others initiate soluble cascades in tissue fluids, and there are large arrays of PRR associated with the cell surface, endomembrane system, or cytoplasm of various cell types. With the cell-associated PRRs, some cell types such as the heterophil, macrophage, B cell, and dendritic cell express a particularly broad array of these receptors. The role for PRRs in immune induction (and direction) cannot be overstated; these are the receptors that kick-start immune responses and the aim of this chapter is to provide an avian-focused overview of PRR.