heimmunesystemisthebody’sdefender.Itidentifiestracksdownanddestroystroublemakersbeforetheycanhurtthebody.Thosetroublemakersmaybeforexamplebacteriafromacutorsplinterameaslesgermacoldbugorevenacancercell.

Theseinvaderstrytotakeoverourtissuesandfeedoffourbodies’nutrients.Iftheysucceedwebecomesickorsometimesevendie.It’sthejobofourimmunesystemstodestroytheseinvadersbeforetheydestroyus.

Sayyoupickedupafluvirus(病毒)lastweek.Perhapsitenteredyourbodythroughacutfromadrinkingglassorfromtheairyoubreathe.Ofcourseyoudidn’tfeelitbutthevirusmadeitswayintoyourbloodstream.Assoonasitenteredyourbodyitbegantoreproduce.Viruseshaveonlyonegoal:totakeoveryourcells.Onceinsideyourbodyvirusestrytoentercellsanddisrupttheirnormalwork.Ifleftalonetheseviruseswouldhurtsomanycellsthatyouwouldweakenorworsegetaseriousillness.

Butthisfluvirusshouldnotbesocomplacentasitseemstobe.Asitreproducesinyourbloodstreamthevirusismetbyacertainkindofwhitebloodcellthelymphocytes.Thelymphocytesarethefootsoldiersthatkeepyoualive.

Thenumberoflymphocytesinyourbodyishardtoimagine.Thousandsofthemcouldfitintheperiodattheendofthissentence.Yourbodyholdsaboutatrillion—that’s1000000000000ofthemorabout3000ineverydropofblood.Sinceyoubeganreadingthissentenceover800000ofthemhavebeencreatedanddestroyed.

Someoftheselymphocytespassthroughasmallwalnut-sizedorgancalledthethymus(胸腺).Thethymusisthebaseoftheneck.Herespecialhormones(荷尔蒙)turnlymphocytesintofightingcellscalledTcells.

Tcellshaveoneterrifictalent:Theycantellthedifferencebetweenfriendandfoe;whatshouldbeinourbodiesandwhatshouldn’t.Theydonotaffectthebody’shealthycells.Yettheyattackeverythingthatisforeigntoourbodiessuchasgermstransplantsandevenourowncellswhichhavebecomeabnormalasinthecaseofcancer.

71.Whatisthefunctionofourimmunesystem?

A.Totakeoverourtissuesandfeedoffourbodies’nutrients.

B.Topreventbacteriaorgermsfromenteringthebody.

C.Todestroythetroublemakersinorderthattheymightnothurtt