So, we just went through the specific biological recognition of the arms of the antibody and I said that this thing was this combination of a smart bomb and a kick me sign. So, it's not just enough to recognize a specific enemy, or what have you, or a specific molecule in order to identify it, I want to have some kind of response to that molecule and to do that, I'm going to turn this thing upside down. Because the part of the antibody that determines biological activity is the FC or stem of the Y of the antibody. We're back to the classes again. We had five different classes and the five different classes have different properties and especially the different G classes have different properties from each other. So, that while this part of the molecule says okay, this is what I want to recognize, this part of the molecule says and this is what I want you to do. You may be most familiar with the specificity of the difference with allergies. An E class antibody that binds to, for example, pollen will specifically summon basophils and activate mast cells and that's how you get that runny nose and that cough. Okay, that seems like a bad thing. How about a good thing? One of the things that the G class and M class antibodies do in particular with this FC stem, is they summon your neutrophils and your macrophages, the phagocytize and eat up the bad guys. So, that's a good function of these things. Another thing that this will do is activate something called complement, and that's the basically the last thing we discussed in part one of this course. But just to anticipate, complement essentially are the landmines in your plasma. What this stem then can do is tell your compliment to attack a particular pathogenic cell. Again, it's a good thing. Now, another thing that you get from this is something called ADCC. So, what we're going to do is essentially put antibodies on a surface of a cell that may have been compromised or maybe a pathogen. So, let's go look at this. Supposing I have a cell, and let's pretend this is a pathogenic cell, and it's some kind of bacterium. You have learned that this bacterium has certain kinds of proteins on its surface. I think that the cell did not come out very well. So, let color and me see if I can darken this. There is the cell membrane and it's got certain kinds of proteins sticking out. All right. What happens is, during the course of the infection, you may learn to identify those proteins and make antibodies against them. So, you will then have some sort of antibody stuck on these guys sticking out and here is the FC part of the antibody. So, here's another antibody attached to this and here's the FC stem sticking out. So, what will happen is that your pathogen or possibly your viral infected cell will have what are these essentially, I would refer to as a kick me sign. So, that's sticking out of these cells is a trigger for your assorted other innate cells to come in here and attack this cell. Now, that could be a macrophage or a neutrophil, then these things are very very helpful in phagocytosis. It could attract, we'll see an NK cell which will come in and kill it in a different way. But in any event, the antibodies here are often M class and G class antibodies that will then essentially direct an attack onto this cell. Now, there is another part of the antibody that we have and this is found in both A class and M class antibodies. These are the antibodies that are held together by the J chain. So, when these antibodies are held together by the J chain, that J chain not only puts them in complexes, it also allows them to cross the membrane. Let me draw quickly the J chain in the form that we're using it to show it on our antibody drawings, and that one is a peptide, that is held together with disulfide bonds. It's not a big peptide, and it has another disulfide bonds sticking out here that attaches to the antibodies. These bonds, then, are part of what trigger the association into a complex, but the part in here actually attaches to a protein in the membranes of the epithelia of cells in the gut. For example, in the lungs sometimes, in breast tissue, in the glands and breast tissue, and allows these antibodies to be secreted. So we have something that enables, something that we call, transcytosis. That is, this little peptide will essentially provide a hook for a membrane embedded transport protein to pick up an antibody complex and transport it from the plasma, across the epithelia and into your gut, or into breast milk. Again, you can do up to 15 grams of this stuff a day, across various membranes in your body, and this is how you protect yourself. So, the last thing we're going to look at is the difference between two words, that you will occasionally encounter. So, we're going to look at the difference between two terms: isotype and idiotype. Isotype and idiotype refer to differences in two different parts of the antibody. So, if I have an antibody here, I will often have two different classes of antibodies that have exactly the same recognition region. For example, I could start out with an antibody that's making an M class, or a B cell making an M class antibody. If a class switches to a G class, it will still be making the same recognition region, and, that is what means, they will have the same idiotype. On the other hand, if we're looking at isotype, we are referring to the FC stem. So, two M class antibodies would have the same isotype, even if they recognize two different antigens, which would give them different idiotypes. Likewise, during class switching, I may make several different kinds of G class antibodies. I might have a G1, a G3, and those would be different isotypes, but if they recognize the same antigen, they would be the same idiotype. My personally, I don't like to use those terms, they're innately confusing, but on the other hand you're going to find that in the literature, you're going to see the words isotype and idiotype. So, while I don't really, I might use them in context, I wouldn't test you on them, I do think that it would be helpful for you not to get confused when you see the term. So, just remember that antibodies are highly varied and therefore, idiosyncratic and varied in this way in what they can bind. However, if you have two of the same kind of exact classes of antibodies, they are isomorphic to each other, and therefore of the same isotype. All right. So, the next thing we're going to look at is the antibodies embedded in a membrane, and the ones that are embedded in a membrane, if they cross-link, as we've seen before, then the cell will begin to produce antibodies. Those antibodies will, by definition, have the same idiotype as the immunoglobulin receptor that linked and promoted their production.
