The phrase “lymph node” refers to specialized tissue sites where T and B cells work cooperatively with each other, to create antibodies that will bind to chunks of proteins that are brought into the lymph nodes, by mobile “antigen-presenting cells.” For historical reasons (described in the downloadable Background section of a patent application which can be found HERE), any cells which can create antibodies are called “B cells.” T cells (which were given that name, because they must pass through the thymus, which is inside the chest, and which is at a halfway point between an organ, and a gland) are the cells which manage, supervise, and control B cells which are trying to create the best possible antibodies. Among other roles, T cells give essential stimulatory molecules to B cells which are creating promising antibody candidates; and, they refuse to give those vital signaling molecules to B cells which are not making promising antibodies, so that the non-promising B cells will die off, and simplify the competition.  
         With that as the definition of “lymph node”, the specialized immune system tissue patches that are exposed and accessible, on the surfaces of mucosal membranes (called MALT patches, for “mucosal associated lymphoid tissues”) fully qualify as “lymph nodes”, since they do indeed contain B and T cells which work together, to create antibodies that will bind to alien/invading/non-self peptides that are presented to them by mobile “antigen-presenting cells”.
         However, MALT patches usually are not called “lymph nodes”. Instead, they belong in a special category, which deserves its own special name, because they have additional roles, capabilities, and functions that internal lymph nodes cannot match.
         And, as brief asides:
         (i)    MALT patches that occur in the nasal cavity or mouth are also called NALT patches, where the N refers to “naso-pharyngeal”, to include not just the nasal cavity, but also the pharynx (i.e., the place where the nasal cavity, mouth, and throat all come together and intersect); and,
         (ii)    MALT patches in the intestines are sometimes called GALT patches (where G stands for “gut”), but they are more commonly called Peyer’s patches.
         The crucial difference between surface-exposed MALT patches, versus “internal” lymph nodes, arises from the presence and activities of highly specialized cells, called “M cells” (from either “membrane” or “microvilli”, depending on what sources you read).
         The “lumenal” (also spelled luminal) surface of an M cell (i.e., the surface which is exposed to food and/or air, in the mouth, nasal cavity, and lungs, and to food which is being digested, in the intestines) will have surface-mounted receptor proteins that are constantly looking for “pathogen patterns”, on particles which contact those receptors; and, when one of those cell receptors recognizes a “pathogen pattern”, on a particle which has contacted the cell, the cell will pull that particle inside the cell, for processing.
         Summarized briefly, “pathogen patterns” (their full scientific name is “pathogen-associated molecular patterns”, or PAMPs) are protein sequences (i.e., specific sequences of the 20 “primary amino acids” which are strung together to make all proteins, in all forms of life on this planet) which appear on numerous different types of microbes. Those “pathogen pattern” sequences became “highly conserved”, and appear on numerous different types of microbes, because they mutated and evolved to a point where they reached a “sweet spot” of truly optimal functionality and efficiency, to a point where any other microbes, carrying different mutated versions of those “highly conserved regions”, will not be able to function as well, and will not be able to compete effectively against microbes having the optimal, highly-conserved sequences.
         Accordingly, if an M cell, on the surface of a MALT patch, detects that a particle which has been inhaled or swallowed has one or more “pathogen patterns” on its surface, the M cell will pull that particle into the cell, using a process called “endocytosis”, or “phagocytosis.” As another brief aside, endocytosis is a broader term, because it also includes a second process called “pinocytosis”, which involves taking in tiny droplets of liquid; however, if intact particles are being pulled in by a cell, the term “phagocytosis” is more specific, and preferred.
         When “phagocytosis” occurs, the cell encloses an incoming particle inside a bubble of membrane material, called a “phagosome”, partly to isolate the cell against the risk of being attacked by the particle, and in many cases, to begin the process of digesting and breaking apart that particle. In nearly all cases involving cells other than M cells, intake of a particle, inside a phagosome, leads to digestion of the particle, by a two-step process: (i) special enzymes will begin pumping acid into the phagosome, to begin softening, prying apart, and loosening up whatever is in the particle, in a manner comparable to way stomach acidity gets chewed food more ready to be digested; then, (ii) a “lysosome” will merge with the phagosome, and that merger will add more acidity, and aggressive digestive enzymes, to the mixture, so that the particle can effectively be “cooked and dissolved” inside a bubble which protects the rest of the cell components from those very harsh conditions.
         However, if phagocytosis by an M cell was triggered by a “pathogen pattern” on a particle which was pulled inside the cell, the M cell will shift into a very different mode, which is believed to be unique to M cells, and only M cells. Using a combination of microtubules and energy-donating enzymes, an M cell with a phagosome containing an apparently dangerous pathogen will actively push, pull, and hustle that phagosome, as rapidly as possible, toward the “bottom surface” membrane (usually called the “basal” membrane) of the M cell. And, when the moving bubble of membrane material (which encloses the phagosome) gets pressed against the basal membrane of the cell, those two membranes will merge together with each other, since they are made of exactly the same types of molecules. When that happens, the “apparently dangerous particle” will be ejected out of the M cell, in “naked form” again, with no membrane surrounding it. That unique M cell process is called “trans-cytosis”, which translates into “through the cell”, or “across the cell”.
         And, that process will eject the particle directly into a special and unusual type of open and roughly U-shaped cavity, on the “underside” (or “basal” side) of the M cell.
         Those cavities are called “docking sites”, because they are functionally similar to the “docking sites” that big trucks will back up into, on the back sides of retail stores, to both: (i) deliver new products to those stores; and, (ii) carry away, for delivery, anything which is too large or heavy for customers in that store to carry away on their own.
         And, those docking sites are extra-special, because that is exactly where large numbers of immature dendritic cells go to, in order to wait for a “pathogen delivery” by an M cell.
         And, that leads into a discussion of “dendritic cells”, on the next page.