Adaptive immune protection of mucous membranes is provided mainly by secretory 
IgA (SIgA) antibodies. This first-line defence is accomplished through an 
ingenious cooperation between the mucosal B-cell system and the epithelial 
glycoprotein called secretory component (SC). This is quantitatively the most 
important receptor of the immune system because it is responsible for external 
transport of locally produced polymeric IgA (pIgA), which is the major humoral 
mediator substance of the whole immune system. Transmembrane SC belongs to the 
Ig supergene family and functions as a general pIg receptor, also translocating 
pentameric IgM externally to form secretory IgM. The B-cells responsible for 
local pIg production are initially stimulated in lymphoepithelial structures, 
particularly the Peyer's patches in the distal small intestine, from which they 
migrate as memory cells to exocrine tissues all over the body. Mucous membranes 
are thus furnished with secretory antibodies in an integrated way, ensuring a 
variety of specificities at every secretory site. There is currently great 
interest in exploiting this integrated or "common' mucosal immune system for 
oral vaccination against pathogenic infectious agents and also to induce 
therapeutic peripheral tolerance to ameliorate T-cell-mediated autoimmune 
diseases. Much remains to be learnt about antigen uptake and processing 
necessary to elicit stimulatory or suppressive mucosal immune responses, and how 
normal homeostasis is maintained in the intestinal mucosa. Considerable 
information has accumulated about various types of immune deviation that may 
lead to local or extraintestinal hypersensitivity reactions against luminal 
antigen, but the crucial mechanisms remain obscure.