Danger-signal molecules capable of activating antigen-presenting cells.

(A) Endogenous danger signals are molecules originating from the host organism; these are products generally released during events of cellular stress. Two general categories of endogenous signals exist: (1) molecules that are secreted by stressed cells such as cytokines, and (2) intracellular products released when membrane disruption occurs (necrosis). For a comprehensive review of these signals and their corresponding receptors, refer to Gallucci and Matzinger.27 (B) Exogenous danger signals include a vast array of molecules associated with pathogenic organisms. For a comprehensive review of these signals and their corresponding receptors, refer to Aderem and Ulevitch.28