Mediates selective transcytosis of polymeric IgA and IgM across mucosal epithelial cells. Binds polymeric IgA and IgM at the basolateral surface of epithelial cells. The complex is then transported across the cell to be secreted at the apical surface. During this process, a cleavage occurs that separates the extracellular (known as the secretory component) from the transmembrane segment. Through its N-linked glycans ensures anchoring of secretory IgA (sIgA) molecules to mucus lining the epithelial surface to neutralize extracellular pathogens. On its own (free form) may act as a non-specific microbial scavenger to prevent pathogen interaction with epithelial cells. Interacts (mainly via CDR1-like domain) with dimeric IgA. Interacts (mainly via CDR2-like domain) with pentameric IgM. Either free or part of the secretory IgA (sIgA) complex that consists of two, four or five IgA monomers, and two additional non-Ig polypeptides, namely the JCHAIN and the secretory component (the proteolytic product of PIGR). Free secretory component interacts with bacterial antigens toxA of C. difficile and eae of E. coli. The Ig-like V-type 1/D1 domain contains three complementarity determining region-like loops CDR1-3, which mediate interaction with IgA and IgM. N-glycosylated. N-glycosylation is required for anchoring IgA molecules to mucus, but is not necessary for Ig binding. Three allotypes are known: allotype T61, allotype T62 and allotype T63. The sequence shown is that of allotype T62.