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　　第??(di)一??(yi)道??(dao)防??(fang)御??(yu)是??(shi)物??(wu)理??(li)屏??(ping)障??(zhang)??，比??(bi)如?(ru)皮??(pi)膚??(fu)和??(he)黏??(nian)膜?(mo)??，像??(xiang)城?(cheng)墻??(qiang)一??(yi)樣??(yang)覆??(fu)蓋??(gai)體??(ti)表??(biao)??，通??(tong)過??(guo)汗??(han)液??(ye)、皮??(pi)脂??(zhi)和??(he)黏??(nian)液??(ye)中??(zhong)的??(de)抗??(kang)菌??(jun)成??(cheng)分??(fen)??，直??(zhi)接??(jie)阻??(zu)擋??(dang)細??(xi)菌??(jun)、病?(bing)毒??(du)入??(ru)侵?(qin)；比??(bi)如??(ru)呼??(hu)吸?(xi)道??(dao)纖??(xian)毛??(mao)??，如??(ru)同??(tong)護??(hu)城??(cheng)河??(he)的??(de)“清??(qing)道?(dao)夫?(fu)”??，通??(tong)過??(guo)擺??(bai)動??(dong)??，將?(jiang)灰??(hui)塵??(chen)和??(he)病??(bing)原?(yuan)體??(ti)排??(pai)出??(chu)體??(ti)外?(wai)??。??　　兒??(er)童??(tong)至??(zhi)青??(qing)少??(shao)年??(nian)期??(qi)(4～18歲??(sui))??，隨??(sui)著??(zhe)免??(mian)疫??(yi)系祝(xi)統(tǒng)??(tong)逐??(zhu)漸??(jian)成??(cheng)熟??(shu)??，適??(shi)應(yīng)?(ying)性??(xing)免??(mian)疫?(yi)(T細??(xi)胞??(bao)、B細??(xi)胞??(bao))開??(kai)始??(shi)發(fā)??(fa)揮??(hui)主??(zhu)力??(li)作??(zuo)用??(yong)??，能??(neng)針??(zhen)對??(dui)病??(bing)原??(yuan)體?(ti)產(chǎn)??(chan)生??(sheng)特??(te)異?(yi)性??(xing)抗??(kang)體??(ti)??。??但??(dan)免??(mian)疫??(yi)系??(xi)統(tǒng)??(tong)也??(ye)可?(ke)能??(neng)因??(yin)“經(jīng)??(jing)驗??(yan)不??(bu)足??(zu)”而??(er)反??(fan)應(yīng)??(ying)過??(guo)度??(du)??，如??(ru)對??(dui)花??(hua)粉??(fen)、食??(shi)物??(wu)等??(deng)無??(wu)害??(hai)物??(wu)質(zhì)??(zhi)過??(guo)度??(du)反??(fan)應(yīng)??(ying)??，引??(yin)發(fā)??(fa)哮??(xiao)喘??(chuan)、濕??(shi)疹?(zhen)；如??(ru)接?(jie)觸??(chu)外??(wai)界??(jie)增??(zeng)多??(duo)??，易??(yi)感??(gan)染?(ran)手??(shou)足?(zu)口?(kou)病??(bing)、水??(shui)痘??(dou)等??(deng)傳??(chuan)染??(ran)病??(bing)??。。