Type II hypersensitivity reactions involve IgG and IgM antibodies directed against cellular antigens, leading to cell damage mediated by other immune system ...SkiptomaincontentMicrobiologyDiseasesoftheImmuneSystemSearchfor:HypersensitivitiesLearningObjectivesIdentifyandcomparethedistinguishingcharacteristics,mechanisms,andmajorexamplesoftypeI,II,III,andIVhypersensitivitiesClinicalFocus:Kerry,Part1Kerry,a40-year-oldairlinepilot,hasmadeanappointmentwithherprimarycarephysiciantodiscussarashthatdevelopswhenevershespendstimeinthesun.Assheexplainstoherphysician,itdoesnotseemlikesunburn.Sheiscarefulnottospendtoomuchtimeinthesunandsheusessunscreen.Despitetheseprecautions,therashstillappears,manifestingasred,raisedpatchesthatgetslightlyscaly.Therashpersistsfor7to10dayseachtime,anditseemstolargelygoawayonitsown.Lately,therasheshavealsobeguntoappearonhercheeksandabovehereyesoneithersideofherforehead.IsKerryrighttobeconcerned,orshouldshesimplybemorecarefulaboutsunexposure?ArethereconditionsthatmightbebroughtonbysunexposurethatKerry’sphysicianshouldbeconsidering?InAdaptiveSpecificHostDefenses,wediscussedthemechanismsbywhichadaptiveimmunedefenses,bothhumoralandcellular,protectusfrominfectiousdiseases.However,thesesameprotectiveimmunedefensescanalsoberesponsibleforundesirablereactionscalledhypersensitivityreactions.Hypersensitivityreactionsareclassifiedbytheirimmunemechanism.TypeIhypersensitivityreactionsinvolveimmunoglobulinE(IgE)antibodyagainstsolubleantigen,triggeringmastcelldegranulation.TypeIIhypersensitivityreactionsinvolveIgGandIgMantibodiesdirectedagainstcellularantigens,leadingtocelldamagemediatedbyotherimmunesystemeffectors.TypeIIIhypersensitivityreactionsinvolvetheinteractionsofIgG,IgM,and,occasionally,IgA1antibodieswithantigentoformimmunecomplexes.Accumulationofimmunecomplexesintissueleadstotissuedamagemediatedbyotherimmunesystemeffectors.TypeIVhypersensitivityreactionsareT-cell–mediatedreactionsthatcaninvolvetissuedamagemediatedbyactivatedmacrophagesandcytotoxicTcell