The mesenteric lymph nodes and Peyer’s patches differentiate independently of the systemic immune system during fetal development, and their development involves distinct chemokines and receptors of the tumor necrosis factor (TNF) family (see Section 9-2). The differences between the GALT and the systemic lymphoid organs are thus imprinted early in life.
In some species such as mice, isolated lymphoid follicles are also found in the lining of the nose, and in the wall of the upper respiratory tract; those in the nose are called nasal-associated lymphoid tissues (NALT), while those in the upper respiratory tract are known as bronchus-associated lymphoid tissues (BALT). The term mucosa-associated lymphoid tissues (MALT) is sometimes used to refer collectively to all such tissues found in mucosal organs, although defined organized lymphoid tissues are not found in the nose or respiratory tract in adult humans unless infection is present.
12-3 The intestine has distinctive routes and mechanisms of antigen uptake.
Antigens present at mucosal surfaces must be transported across an epithelial barrier before they can stimulate the mucosal immune system. Peyer’s patches and isolated lymphoid follicles are highly adapted for the uptake of antigen from the intestinal lumen. The M cells in the follicle-associated epithelium are continually taking up molecules and particles from the gut lumen by endocytosis or phagocytosis (see Fig. 12.7). For several bacteria this may involve specific recognition of the bacterial FimH protein found in type 1 pili by a glycoprotein (GP2) on the M cell. This material is transported through the interior of the cell in membrane-bound vesicles to the basal cell membrane, where it is released into the extracellular space—a process known as transcytosis. Because M cells lack a glycocalyx and so are much more accessible than enterocytes, a number of pathogens target M cells to gain access to the subepithelial space, even though they then find themselves in the heart of the intestinal adaptive immune system. These include Salmonella enterica serotype Typhi, the causative agent of typhoid fever; other Salmonella enterica serotypes, which are major causes of bacterial food poisoning; Shigella species that cause dysentery; and Yersinia pestis, which causes plague. Poliovirus, reoviruses, some retroviruses such as HIV, and prions such as the causal agent of scrapie follow the same entry route. After entry into the M cell, bacteria produce proteins that reorganize the M-cell cytoskeleton in a manner that encourages their transcytosis.