Unifying external and internal immune defences.

External immune defence, such as antimicrobial secretions, is not generally viewed as part of the immune system. Nevertheless, it constitutes a first barrier to pathogens and manipulates the microbial environment. Hygienic measures from the protection of oneself or conspecifics, of the nesting site, or of stored food might be more efficient with secreted antimicrobials. Here, we argue that antimicrobial secretions represent an extended arm of the immune system, forming an underappreciated selective force in the evolution of immune systems. Integrating external immunity into the immune system and general host physiology provides an amenable concept for the understanding of immune system variation and life-history trade-offs. Future research should evaluate complementary or additive roles of antimicrobial secretions in relation to internal immunity.