Fish, like all living organisms, have mechanisms for recognizing and responding to non-self entities, such as pathogens. One key mechanism for recognition of non-self in fish is through the use of pattern recognition receptors (PRRs). These receptors can recognize and bind to specific molecular patterns found on the surface of pathogens, such as lipopolysaccharides (LPS) on the cell walls of gram-negative bacteria, or double-stranded RNA (dsRNA) found in some viruses. Once a pathogen is recognized by a PRR, it triggers a cascade of immune responses including the activation of the complement system, the production of antibodies, and the activation of phagocytes.