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　　嬰??(ying)幼?(you)兒??(er)期??(qi)(0～3歲??(sui))??，新??(xin)生??(sheng)兒??(er)免?(mian)疫??(yi)系??(xi)統(tǒng)??(tong)像??(xiang)一??(yi)支??(zhi)未??(wei)經(jīng)??(jing)實(shí)?(shi)戰(zhàn)??(zhan)的??(de)“新??(xin)兵??(bing)部??(bu)隊(duì)??(dui)”??，先??(xian)天??(tian)性??(xing)免??(mian)疫??(yi)(如??(ru)中??(zhong)性?(xing)粒??(li)細(xì)??(xi)胞??(bao)、巨??(ju)噬??(shi)細(xì)??(xi)胞??(bao))活??(huo)性??(xing)高??(gao)??，但??(dan)皮??(pi)膚??(fu)、黏?(nian)膜??(mo)屏??(ping)障??(zhang)薄??(bo)弱??(ruo)??，易??(yi)受??(shou)病??(bing)毒?(du)(如??(ru)呼??(hu)吸??(xi)道?(dao)合??(he)胞??(bao)病??(bing)毒??(du)、輪??(lun)狀??(zhuang)病??(bing)毒??(du))侵??(qin)襲??(xi)??。??此??(ci)時(shí)??(shi)主??(zhu)要??(yao)依??(yi)賴?(lai)母??(mu)體??(ti)通??(tong)過??(guo)胎??(tai)盤??(pan)和??(he)母??(mu)乳??(ru)傳??(chuan)遞??(di)的??(de)抗??(kang)體??(ti)(如??(ru)IgG、IgA)提??(ti)供??(gong)保??(bao)護(hù)??(hu)??。。