GeneralPropertiesofImmuneResponsesThetermimmunityisderivedfromtheLatinwordimmunitas,whichreferredtotheprotectionfromlegalprosecutionofferedtoRomansenatorsduringtheirtenuresinoffice.Historically,immunitymeantprotectionfromdiseaseand,morespecifically,infectiousdisease.Thecellsandmoleculesresponsibleforimmunityconstitutetheimmunesystem,andtheircollectiveandcoordinatedresponsetotheintroductionofforeignsubstancesiscalledtheimmuneresponse.Thephysiologicfunctionoftheimmunesystemisdefenseagainstinfectiousmicrobes.However,evennoninfectiousforeignsubstancescanelicitimmuneresponses.Furthermore,mechanismsthatnormallyprotectindividualsfrominfectionandeliminateforeignsubstancesarethemselvescapableofcausingtissueinjuryanddiseaseinsomesituations.Therefore,amoreinclusivedefinitionofimmunityisareactiontoforeignsubstances,includingmicrobes,aswellastomacromoleculessuchasproteinsandpolysaccharides,regardlessofthephysiologicorpathologicconsequenceofsuchareaction.Immunologyisthestudyofimmunityinthisbroadersenseandofthecellularandmoleculareventsthatoccurafteranorganismencountersmicrobesandotherforeignmacromolecules.HistoriansoftencreditThucydides,inAthensduringthefifthcenturyBC,ashavingfirstmentionedimmunitytoaninfectionthathecalled“plague”(butthatwasprobablynotthebubonicplaguewerecognizetoday).Theconceptofimmunitymayhaveexistedlongbefore,assuggestedbytheancientChinesecustomofmakingchildrenresistanttosmallpoxbyhavingtheminhalepowdersmadefromtheskinlesionsofpatientsrecoveringfromthedisease.Immunology,initsmodernform,isanexperimentalscience,inwhichexplanationsofimmunologicphenomenaarebasedonexperimentalobservationsandtheconclusionsdrawnfromthem.Theevolutionofimmunologyasanexperimentaldisciplinehasdependedonourabilitytomanipulatethefunctionoftheimmunesystemundercontrolledconditions.Historically,thefirstclearexampleofthismanipulation,andonethatremainsamongthemostdramaticeverrecorded,wasEdwardJenner’ssuccessfulvaccinationagainstsmallpox.Jenner,anEnglishphysician,noticedthatmilkmaidswhohad...
