Hypersensitivity reactions occur when the normally protective immune system responds abnormally, potentially harming the body.ExpandallsectionsRegister/LoginTrustedmedicalexpertiseinseconds.Access1,000+clinicalandpreclinicalarticles.Findanswersfastwiththehigh-poweredsearchfeatureandclinicaltools.Tryfreefor5daysEvidence-basedcontent,createdandpeer-reviewedbyphysicians.Readthedisclaimer.HypersensitivityreactionsLastupdated:June29,2021SummaryHypersensitivityreactionsoccurwhenthenormallyprotectiveimmunesystemrespondsabnormally,potentiallyharmingthebody.Variousautoimmunedisordersaswellasallergiesfallundertheumbrellaofhypersensitivityreactions,thedifferencebeingthatallergiesareimmunereactionstoexogenoussubstances(antigensorallergens),whereasautoimmunediseasesarisefromanabnormalimmuneresponsetoendogenoussubstances(autoantigens).Asymptomaticreactiononlyoccursinsensitizedindividuals,i.e.,theymusthavehadatleastonepriorasymptomaticcontactwiththeoffendingantigen.Hypersensitivityreactionsarecommonlyclassifiedintofourtypes.TypeIhypersensitivityreactionsareimmediateallergicreactions(e.g.,foodandpollenallergies,asthma,anaphylaxis).TypeIIhypersensitivityreactionsarereferredtoascytotoxic,astheyinvolveantibodiesthatarespecifictoparticulartissueswithinthebodyandcausedestructionofcellsinthesetissues(e.g.,autoimmunehemolyticanemia,Goodpasturesyndrome).TypeIIIhypersensitivityreactionsareimmunecomplex-mediated,withtissuedamagecausedbyantigen-antibodycomplexdeposition(e.g.,manyvasculitidesandglomerulonephritides).TypeIVhypersensitivityreactions(e.g.,TBskintests,contactdermatitis)aredelayedandcell-mediatedandaretheonlyhypersensitivityreactionthatinvolvessensitizedTlymphocytesratherthanantibodies.Unliketruehypersensitivityreactions,whichoccuraftersensitization,nonallergichypersensitivityreactions(e.g.,pseudoallergies)causemastcellactivationandhistaminereleaseafterinitialexposuretoatriggersubstance(e.g.,radiocontrastmedia).Seealso“Anaphylaxis.”OverviewDefinitionsHypersensitivityreaction:acondi