Vertebrate animals have two entirely different types of antibodies, to help them fight off pathogens and diseases.

          Actually, vertebrates have six distinct type of antibodies, but most of them are not important, in actually helping fight off pathogens. So-called "IgE" antibodies are generally unwanted, and are more involved in allergic reactions, than in defending against diseases. In a classic goof-up, the "IgM" name applies to two very different types of antibodies: (i) an extra-large ring structure, formed by coupling together 5 Y-shaped antibodies; and, (ii) a "small, inexpensive, trial-sized" antibody, created only inside lymph nodes, when B cells are creating new candidate antibodies in response to a newly-presented antigen sequence (those extra-small "testing" versions do not leave a lymph node, and are eliminated once a T cell chooses a "winning" B cell and signals it to begin making the full-sized versions). And, a rare type of antibody is called IgD, because it has a "delta" chain. Anyone who wants more info on (or pictures of) any of those can find that info easily, in Wikipedia or via an internet search.

          So, that leave two main types of antibodies which actively help fight off pathogens and diseases. The easiest way to grasp a huge and crucially important difference between them, is to realize that:

          (i) the "famous" type, usually called IgG ("immunoglobulin Gamma"), which essentially all educated people already know about, have shapes that roughly resemble the letter Y, because they have two "sticky arm" components, attached to a single "stem" component. Here are two ways to illustrate an IgG antibody:

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          These Y-shaped antibodies can work effectively, only INSIDE the body, because they do not carry any toxins, or defensive weapons, and they are much too small (as a single molecule) to be able to seriously entangle, slow down, or hinder even a virus, let alone a bacterial cell. So, they function by changing the shapes of their “stem portions”, when either of the two “sticky arms” binds to something that looks dangerous.

          We call that process a “tag and flag” process; the antibody effectively tells an apparent pathogen, "Aha! I found you, and now I've caught you! And so, TAG! You are now IT!" The sticky arm(s) of the IgG will latch onto the particle, and when the stem portion of that antibody changes shape, it effectively becomes a "flag", attached to that particle, which begins the process of alerting the immune system that an invading foreign particle has been identified, latched onto by an antibody, and marked.

           Next, a special class of proteins (called “complement proteins”) will recognize and bind to a “shape-shifted” stem on an internal IgG antibody. That binding reaction will trigger an "activation cascade" which will cause the complement proteins to have a "protective cap portion" removed, to expose an active portion. When complement proteins become “activated” – by binding to an IgG antibody which has bound to something – they perform the equivalent of attaching a loud electronic alerting device, with a loud siren and a flashing light, to the shape-shifted stem of an IgG antibody. Any nearby immune cells will recognize and respond to those signals, and they will go to where the shape-shifted antibody, and the complement proteins, have attached to the alien/hostile/non-self/intruding particle.

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          However, in “secreted mucosal fluids” which are completely outside of any cells or tissues (those fluids include saliva, nasal mucus, digestive juices, lung fluids, fluids in the vaginal cavity and urethra, and fluids that keep the eyes lubricated), there simply are not enough immune cells (or complement proteins) present, in those fluids, to be able to offer any significant help, to any “shape-shifted” antibodies.

          Therefore, in a beautiful, brilliant, and elegant feat of evolution, the immune systems of vertebrates somehow figured out (oe "evolved with") a way to strap two antibodies to each other, via their stem components, to create elongated antibody “dimers”, with each “dimer” containing two Y-shaped antibodies coupled to each other, so that each and every dimer contains not just two, but four “sticky arms” (with two at each end of a double-length molecule). Those dimers are called “secreted IgA antibodies” (or related phrases, such as “sIgA dimers”). Here is an illustration of an IgA dimer:

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          Since a “tag ang flag” function is useless, in secreted mucosal fluids with no immune cells around, the MUCOSAL immune system developed a “grab and drag” process instead. If even a single sticky arm segment can latch onto some foreign particle, the antibody will effectively block and prevent that particle from burrowing, tunneling, or otherwise penetrating into any mucosal cells or membranes. Instead, the IgA antibody will forcibly drag that particle down into the stomach acids (which will kill nearly all microbes), if they connected inside the mouth or nasal cavity; or, the antibody will keep that particle suspended in the liquefied and then semi-solid mass of food that is passing through the intestines, until the particle and the antibody get “pooped out” and eliminated from the body; or, if they connected inside the windpipe, bronchial tubes, or lungs, they will be coughed up, and either spit out, or swallowed.

          Most people have never even heard of “secreted mucosal antibody dimers”, and they have no idea that they even exist. However, here is an absolutely crucial fact, which can help people better understand how important they  are:

 

In just the 3 to 5 pounds of secreted mucosal fluids that a typical human adult is carrying, at any given moment, there are MORE THAN TWICE AS MANY SECRETED IgA ANTIBODIES, THAN ALL OF THE Y-SHAPED IgG ANTIBODIES IN THE ENTIRE REMAINING BUlK AND WEIGHT, OF THAT PERSON.

 

          In summary, because of the factors described in Topic 1, the number of pathogens that infect animals by penetrating into a “mucosal membrane” is hundreds of times larger and more populous than the second-largest category of pathogens (i.e., “blood-borne" pathogens, usually transmitted by insect bites). So, driven by constant and pressing needs, mammals developed a specialized “mucosal” immune system, which operates in ways that are almost entirely separate from, and independent of, the internal immune system. The immune system figured out how to create IgA dimer antibodies, and how to secrete them into the mucosal fluids that are no longer inside any cells or tissues (for anyone interested in more details about HOW they are made and secreted, in ways that go deeper than the figure caption above, download the patent application section available from this website, and search for the words "J chain" or "secretory component"). Then, once those sIgA antibody dimers have been created and secreted, they are entirely on their own, and they will get no more help from any other immune cells or components.

          However, there are two unhappy and difficult facts, about injectable vaccines, which everyone should know about.

          The first unhappy fact is that injected vaccines do not trigger mucosal antibody formation, except perhaps as rare, sporadic, and unreliable side effects. As described on the next page, there simply has not been any good way, before now, to create vaccines that can safely and reliably trigger mucosal antibody responses, against any antigen sequences that are "loaded onto" those vaccine particles.

          The second unhappy fact is this: in order to make injected vaccines more potent and effective, the complete injectable formulations must include harsh and unpleasant chemical additives, called "adjuvants". The reason why adjuvants must be "necessarily nasty" compounds, which are used to deliberately irritate and inflame the cells and tissues at the site of an injection, are described on the next page. And, it is hoped and believed that the "MALT-targeting" approach to creating mucosal vaccines can entirely bypass, and eliminate, any need to include any harsh, inflammatory "adjuvants" in the final formulations. If that can be accomplished, it will provide a major and much-appreciated step forward, to a point where MALT-targeting vaccines might earn and deserve the label of "Post-Adjuvant Vaccines".