Medical Microbiology

Host–PathogenInteractions 7

doplasmicreticulum,pseudopods,flagella,cilia,kinetoplasts,etc.Manypara-

siticprotozoaaretransmittedbyarthropods,wherebymultiplicationand

transformationintotheinfectiousstagetakeplaceinthevector.

Animals.....................................................

&Helminths.Parasiticwormsbelongtotheanimalkingdom.Theseare

metazoanorganismswithhighlydifferentiatedstructures.Medicallysignif-

icantgroupsincludethetrematodes(flukesorflatworms),cestodes(tape-

worms),andnematodes(roundworms).

&Arthropods.Theseanimalsarecharacterizedbyanexternalchitinskele-

ton,segmentedbodies,jointedlegs,specialmouthparts,andotherspecific

features.Theirroleasdirectcausativeagentsofdiseasesisaminorone

(mites,forinstance,causescabies)ascomparedtotheirroleasvectorstrans-

mittingviruses,bacteria,protozoa,andhelminths.

Host–PathogenInteractions.........................................

&Thefactorsdeterminingthegenesis,clinicalpictureandoutcomeofan

infectionincludecomplexrelationshipsbetweenthehostandinvadingor-

ganismsthatdifferwidelydependingonthepathogeninvolved.Despite

thisvariability,anumberofgeneralprinciplesapplytotheinteractionsbe-

tweentheinvadingpathogenwithitsaggressionfactorsandthehostwithits

defenses.Sincethepathogenesisofbacterialinfectiousdiseaseshasbeenre-

searchedverythoroughly,thefollowingsummaryisbasedonthehost–in-

vaderinteractionsseeninthistypeofinfection.

Thedeterminantsofbacterialpathogenicityandvirulencecanbeoutlinedas

follows:

&Adhesiontohostcells(adhesins).

&Breachingofhostanatomicalbarriers(invasins)andcolonizationoftis-

sues(aggressins).

&Strategiestoovercomenonspecificdefenses,especiallyantiphagocytic

mechanisms(impedins).

&Strategiestoovercomespecificimmunity,themostimportantofwhichis

productionofIgAproteases(impedins),molecularmimicry,andimmunogen

variability.

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Kayser, Medical Microbiology © 2005 Thieme