4.DiseasesoftheImmuneSystemTheimmunesystemisacomplexandhighlydevelopedsystem,yetitsmissionissimple:toseekandkillinvaders.Ifapersonisbornwithaseverelydefectiveimmunesystem,deathfrominfectionbyavirus,bacterium,fungusorparasitewilloccur.Inseverecombinedimmunodeficiency,lackofanenzymemeansthattoxicwastebuildsupinsideimmunesystemcells,killingthemandthusdevastatingtheimmunesystem.AlackofimmunesystemcellsisalsothebasisforDiGeorgesyndrome:improperdevelopmentofthethymusglandmeansthatTcellproductionisdiminished.Mostotherimmunedisordersresultfromeitheranexcessiveimmuneresponseoranautoimmuneattack.Asthma,familialMediterraneanfeverandCrohnsdisease(inflammatoryboweldisease)allresultfromanover-reactionoftheimmunesystem,whileautoimmunepolyglandularsyndromeandsomefacetsofdiabetesareduetotheimmunesystemattackingselfcellsandmolecules.Akeypartoftheimmunesystemsroleistodifferentiatebetweeninvadersandthebodysowncells-whenitfailstomakethisdistinction,areactionagainstselfcellsandmoleculescausesautoimmunedisease.SuchsuchasAsthma，Ataxiatelangiectasia，Autoimmunepolyglandularsyndrome，Burkittlymphoma，Diabetes,type1，DiGeorgesyndrome，FamilialMediterraneanfever，Immunodeficiencywithhyper-IgM，Leukemia,chronicmyeloidandSeverecombinedimmunodeficiencyetc.