Innate humoral defense factors

Innate defense mechanisms help to define the minimal requirements for successful colonization by commensal and pathogenic organisms-those that cannot adapt to these conditions will be incapable of maintaining themselves within the host. Phagocytes represent a major component of innate defense at the cellular level, and all classes of phagocytes, including macrophages, neutrophils, eosinophils, and mast cells, occur within the mucosal tissues. Although the activities of phagocytes may be largely confined to the tissues themselves rather than taking place within the lumen-the microenvironment close to the mucosal surface may permit phagocytic activity. As most secretions are hypotonic, phagocytes probably do not survive with functional activity for long in the bulk fluid phase. Innate immune recognition is mediated by a series of germline-encoded receptors, known as pattern-recognition receptors (PRRs)-which detect virulent microorganisms through recognition of invariant pathogen-associated molecular patterns (PAMPs). Studies in wild-type and PRR-deficient mice as well as human macrophages activated by lipopolysaccharide (LPS), taxol, or bacterial fimbriae, suggest that-TLRs, CD14, and β₂ integrins form a functional multireceptor complex that coordinates induction of intracellular signals.