The notion of danger

Immunologists have traditionally focused on the adaptive immune response, a nd have considered that its main function is to distinguish between self (e ndogenous) and nonself (foreign) antigens. In contrast, a number of researc hers have recently proposed that the main function of the immune system is to eliminate microbial pathogens. The innate immune system, largely neglect ed in the past, plays an essential role in initiating anti-microbial respon ses. This system reacts rapidly against infections, and in parallel, it act ivates and influences the adaptive system. Several theories have been propo sed to explain how the innate and adaptive immune systems interact during t he immune response and affect immune tolerance. According to the model of " immune danger ", any agent capable of destroying tissue through necrosis a nd inflammation can trigger an immune response. The " danger model " has re cently converged with a second one, which proposes that the innate system r ecognizes microorganisms mainly through specific structural motifs expresse d by pathogens but not by the host. In contrast, a competing model claims t hat activation of the immune system is determined by the location and the d ose of the antigen, and by the duration of antigen exposure. All three mode ls often invoke microbial pathogens (known or still unknown) to account for autoimmune pathologies.