A pattern recognition receptor (PRR), also called a primitive pattern recognition receptor or a pathogen recognition receptor, is a type of protein that interacts with the innate immune system. The purpose of pattern recognition receptors is to identify the molecular patterns of pathogens and antigens. These patterns are unique to foreign microorganisms that are not related to the cells in the body. They can also detect patterns caused by damage-associated molecules, such as the patterns from injured cells.
The innate immune system functions differently than the adaptive immune system. Adaptive immune responses are not immediately triggered and can take days to protect the body against newly discovered antigens. Innate immune responses occur when an antigen invasion has been detected. This part of the immune system does not detect every single invading antigen. Instead, patterns of specific pathogens and antigens are detected by a pattern recognition receptor.
Molecules in cells have molecular patterns unique to specific groups. For example, gram positive bacteria contain lipotechoic and peptidoglycan acids within the cell wall, and gram negative bacteria contain lipopolysaccharide (LPS) within the cell wall. Both of these types of bacteria can be detected by a pattern recognition receptor. These receptors detect the patterns and trigger the innate immune system. There are two types of pattern recognition receptors: endocytic and signaling.
An endocytic pattern recognition receptor is the first of the two types of receptors. Endocytic PRRs are located on phagocyte surfaces. Phagocytes are cells, such as white blood cells, that work to remove unwanted substances. The PRRs attached to these cells to trigger the binding of phagocytes and microorganisms. Once the pathogens or antigens are bound to the phagocytes, destruction and removal begin.
Within the group of endocytic PRRs, there are sub-groups. A mannose pattern recognition receptor binds to mannose-rich microorganisms. Scavenger PRRs target cell walls of bacteria. Opsonin PRRs bind phagocytes to microbes. N-formyl receptors are triggered by the bacteria that produce the N-formyl methionine protein.
A signaling pattern recognition receptor, the second type of receptor, binds microbial molecules and receptors together. These receptors recognize pathogens or antigens based on patterns of molecular activity. This binding triggers the production of cytokines. In the immune system, cytokines can trigger both the innate and adaptive immune system.
Signaling PRRs are split into sub-groups based on where the cell receptors are located in a cell. A signaling pattern recognition receptor can be on the surface of immune cells or within cell membranes. These receptors can also be located in the cytoplasm or secreted in tissue fluids and plasma.