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chapter1IHistoricalPerspectiveIInnateImmunityIAdaptiveImmunityIComparativeImmunityIImmuneDysfunctionandItsConsequencesNumerousTLymphocytesInteractingwithaSingleMacrophageOverviewoftheImmuneSystemTdefensesystemthathasevolvedtoprotectanimalsfrominvadingpathogenicmicroorganismsandcancer.Itisabletogenerateanenormousvarietyofcellsandmoleculescapableofspecificallyrecognizingandeliminat-inganapparentlylimitlessvarietyofforeigninvaders.Thesecellsandmoleculesacttogetherinadynamicnetworkwhosecomplexityrivalsthatofthenervoussystem.Functionally,animmuneresponsecanbedividedintotworelatedactivities—recognitionandresponse.Immunerecognitionisremarkableforitsspecificity.Theimmunesystemisabletorecognizesubtlechemicaldifferencesthatdistinguishoneforeignpathogenfromanother.Further-more,thesystemisabletodiscriminatebetweenforeignmoleculesandthebody’sowncellsandproteins.Onceafor-eignorganismhasbeenrecognized,theimmunesystemrecruitsavarietyofcellsandmoleculestomountanappro-priateresponse,calledaneffectorresponse,toeliminateorneutralizetheorganism.Inthiswaythesystemisabletoconverttheinitialrecognitioneventintoavarietyofeffectorresponses,eachuniquelysuitedforeliminatingaparticulartypeofpathogen.Laterexposuretothesameforeignorgan-isminducesamemoryresponse,characterizedbyamorerapidandheightenedimmunereactionthatservestoelimi-natethepathogenandpreventdisease.Thischapterintroducesthestudyofimmunologyfromanhistoricalperspectiveandpresentsabroadoverviewofthecellsandmoleculesthatcomposetheimmunesystem,alongwiththemechanismstheyusetoprotectthebodyagainstforeigninvaders.Evidenceforthepresenceofverysimpleimmunesystemsincertaininvertebrateorganismsthengivesanevolutionaryperspectiveonthemammalianimmunesystem,whichisthemajorsubjectofthisbook.El-ementsoftheprimitiveimmunesystempersistinverte-bratesasinnateimmunityalongwithamorehighlyevolvedsystemofspecificresponsestermedadaptiveimmunity.Thesetwosystemsworkinconcerttoprovideahigh...