How is the innate immune system activated?
Innate immunity is activated by host signals called alarmins or pathogen signals called pathogen-associated molecular patterns (PAMPs). Alarmins are endogenous molecules that are constitutively available and passively released from necrotic cells upon infection or tissue injury. They are also secreted by stimulated leukocytes and epithelia. PAMPs are molecules produced only by microbes. Both alarmins and PAMPs activate innate immunity through pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs). Alarmins and PAMPs activate and recruit cells of the innate immune system including dendritic cells (DCs).