多聚免疫球蛋白受体（pIgR）在粘膜免疫中的重要功能

多聚免疫球蛋白受体（pIgR）属于Ⅰ型跨膜糖蛋白，可与多聚免疫球蛋白A和多聚免疫球蛋白M特异性结合，通过穿胞转运，将它们从上皮细胞基底侧膜转运到顶膜，并最终分泌到外分泌液中去. 在此过程中，多聚免疫球蛋白受体的细胞外段被水解，释放出与多聚免疫球蛋白A或多聚免疫球蛋白M相结合的细胞外段（又称为分泌成分）. 分泌成分是sIgA分子的重要组成部分，直接参与sIgA的粘膜防御功能，而且在被动粘膜免疫中也有重要作用. 多聚免疫球蛋白受体通过介导细胞内多聚免疫球蛋白的转运，可以在粘膜的腔面阻止病原体粘附，在上皮细胞内中和病毒，也可以将固有层内的抗原分泌出去. 因此，多聚免疫球蛋白受体的有效分泌是多聚免疫球蛋白发挥粘膜防御功能的必要条件. 但在某些情况下，该受体也可以介导微生物对上皮屏障的入侵. 多聚免疫球蛋白受体是高度 N -糖基化的，其分子中独特的糖链结构，可能与受体的穿胞转运、sIgA在粘膜的正确定位，以及抗原对上皮细胞的粘附有关. 多聚免疫球蛋白受体和分泌成分参与的多重分子机制，使它们在粘膜免疫中起着举足轻重的作用.

Important Role of Polymeric Immunoglobulin Receptor in Mucosal Immunity

Polymeric immunoglobulin receptor (pIgR) is a kind of type Ⅰ trans-membrane glycoprotein， which could transport via transcytosis from the basal membrane to the apical membrane of epithelia by specifically binding to polymeric IgA or polymeric IgM. At last,the complex of pIgR-polymeric IgA or pIgR-polymeric IgM is released into the secretion as sIgA or sIgM. During this process, the extracellular domain of pIgR is cleaved and released as secretory component(SC) together with polymeric IgA or polymeric IgM. SC is not only the important component of sIgA which directly participate in the mucosal protect function of sIgA, but also plays an important role in the passive mucosal immunity. pIgR can mediate the transcytosis of polymeric IgA,and thus ensures the multiple functions of sIgA, such as luminal prevention of pathogen attachment, intracellular neutralization of viruse and secretion of antigen from lamina propria. Thus, the efficient secretion of pIgR is a prerequisite of polymeric IgA-mediated mucosal protection. However,it seems that pIgR sometimes permits pathogens to invade the epithelial barrier. pIgR is highly N-glycosylated, the unique glycans, which may play a role in the transcytosis of this receptor. Furthermore, these glycans may insure the correct localization of sIgA on mucosa or facilitate the adhering of antigen to epithelial cells. The involvement of multiple molecular mechanisms indicates the influential roles of pIgR and SC im mucosal immunity.