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In order to survive in a world laden with microorganisms, most multi-cellular organisms ought to depend on a network of host defense mechanisms which in most cases, involves several levels of interacting systems. Since the initial contact of fastidious microorganismswith the host usually occurs at inner or outer body surfaces, they should be the primary site for an immune reaction to occur. Thus, innate immune responses refer to the first line of host defense, which acts within a few hours after microbial exposure to mucosal surfaces. Upon recognition of conserved molecular microbial patterns such as PAMs or Pathogen-Associated Molecular Patterns (e.g. LPS and cell wall components) and Toll-like receptors (TLR) (Hoffman et al. 1999; Aderem and Ulevitch, 2000; Akira et al. 2001) initiate the immune responses of the host. Using the urinary and gastro-intestinal tract as model systems, information has been obtained on how organ- and cell-specific expression patterns of TLR on epithelial cells correlate to the ability of an organ to rapidly respond to bacterial infections has been obtained. It has become clear now that understanding the innate response to pathogens will certainly provide insights to host defenses as well as the strategies used by pathogens to circumvent these defense mechanisms. Remarkably, the pattern-specific recognition system already acknowledged in animals, has also been reported in plants (Dangl and Jones, 2001).

In complex system suchas humans, an invading microorganism can simply be eliminated by this primary reaction - the innate response - without requiring an activation of the adaptive immunity, the next step in this complex cascade . If the invading microbe outgrows the innate host defence, endogenous effector mechanisms of the innate system are up-regulated and have direct antimicrobial activity and mediator function to attract inflammatory cells and cells of the adaptive immune system. In lower eukaryotes, mostly invertebrates, the adaptive system is nonexistent, thus accounting for the versatile and effective role the innate system has in order to control, by itself, the invasiveness of a given pathogen reviewed by (Otvos, 2000).

HOFFMANN, J.A.; KAFATOS F.C.; JANEWAY C.A. and EZEKOWITZ R.A. Phylogenetic perspectives in innate immunity. Science, 1999, vol. 284, no. 5418, p. 1313-1318.

ADEREM, A. and ULEVITCH, R. Toll-like receptors in the induction of the innate immune response. Nature, 2000,vol. 406, no. 6797, p. 782-787.

AKIRA, S.; TAKEDA, K. and KAICHO, T. Toll-like receptors: Critical proteins linking innate and acquired immunity. Nature Immunology, 2001, vol. 2, no. 8, p. 675-680.

DANGL, J.L. and JONES, J. D. Plant pathogens and integrated defense responses to infection. Nature, 2001, vol. 411, no. 6839, p. 826-833.

OTVOS, L. Jr. Antibacterial peptides isolated from insects. Journal of Peptide Sciences, 2000, vol. 6, no. 10, p. 497-511.