This lec section is really just a brief introduction to all of the strange step we're going to be looking at for the rest of the course. So, it's a bit of a fast-track through a vocabulary, introduction and a little bit of a reminder of maybe where to find some organizational help. The first thing I wanted to remind you of is that when we're looking at the adaptive immune system, we're looking at vertebrates. So, this is a frog, and it may not have the complete content adaptive systems we see in reptiles, birds, and mammals, but it does have the ability to change its genes in response to environmental stresses. So, if we're looking at the adaptive system, however, one of the things that we really need to remember is that the innate system predated it and the adaptive system was basically piggybacked on top of it in some very ancient fish, and that today, these two systems constantly talk back and forth to each other. So, while you often see innate and adaptive systems being described as being distinct systems, one thing that you need to remember is that there is constant crosstalk between every aspect of the immune system and in particular the innate system communicates with the adaptive, and the adaptive communicates with the innate. Having said that, here is a reminder of one of the more important in a function's phagocytosis. So, if we're going to look at these two systems, maybe a little bit in isolation, here is a reminder that what we mean by innate are things that are in the ready, ready to go, don't have to rearrange genes are there waiting for a problem. In many cases, your neutrophils, or I guess this one is a macrophage, are waiting and will phagocytize and kill pathogens before they have a chance to come in and divide and start to cause a problem. Other things like NK cells and additional other cell types which we'll cover in the next lecture, also function in this respect. So, in addition to this phagocytosis process, we also have proteins that are just simply waiting for the ability to recognize pathogens, that includes complement, include things like lysozyme, which are enzymes that break up bacteria, and your physical barriers including your epithelia and particular your skin are also designed to prevent an infection from ever getting into you. So, all of these can be considered innate functions, okay? They recognize patterns that indicate you've got a pathogen, you're born with a functional genes that code for them, and in fighting these guys off, they often get information that they then pass on to the adaptive immune system that helps to design your effective adaptive response. So, here's a quick sort of table, just to put it all together, your innate response is going to be fast. It's always there. It recognizes patterns as the cells I was talking about. Now, if it's going to communicate and up-regulate the adaptive immune system, the first time the adaptive immune system gets up and running, it's going to take it a full two weeks before it's functional. The memory cells though, will be up and running within three days on a subsequent exposure. One of the reasons this take so long is, you have to rearrange genes and then select for whatever components these genes produce that will work. So, therefore, you can recognize a specific part of proteins. So, what we're going to be looking at is primarily B cells and T cells and the antibodies that are produced by these cells. So, here is one way of looking at it. Here is a T-helper cell and that's really as it turns out is at the center of the adaptive response. Notice however, that an antigen presenting cell is stimulating this T cell and this antigen presenting cell is part of your innate immune system. It could be a macrophage, it could be a dendritic cell. Now, the T-helper cell then will stimulate B cells. It'll stimulate Tc, or killer T cells, CTLs and it goes right back if you see here and stimulates macrophages which are an important part of the innate immune system. So, here you can sort of see the interaction of a number of types of the immune system kind of laid out for you here. So, what we're talking about then, are these adaptive cells that will be up-regulated to produce the specific response. Now, here is an outline of the B cell response, which is called a humoral response. Now, personally, I find the term humoral response to be sort of insulting. It's really an ancient classical Greek and Roman term humor, that refers to the four fluids of the body: black bile, yellow bile, phlegm and the blood. So, in the humor, in the blood there was soluble immune functions and it turns out they're just antibodies. So, when we're talking about humoral immunity, we're talking about the production of antibodies, that's a B cell function. Here, you can see the B cells are maturing and are going to produce antibodies which they will release into the blood. Those antibodies will be developed because they are highly specific for a particular shape or epitope on a protein. This is something that is produced by what we call clonal expansion. You make a whole bunch of different kinds of B cells with different antibodies and you will cause the cells to divide only if they're making a useful antibody. So, that's going to be a TH cell function. Now, in the cell mediated responses here, we have a cytotoxic T cell. If you can see, this immature T cell is also recognizing antigen. In this case, it's antigen that's produced inside of an infected cell. It is recognizing this antigen as being foreign. When it does that, it will interact with a TH cell, it will mature and it will become a cytotoxic TC cell. Now, if you'll notice, here's the CD8. That's going to be important in binding to the MHC1. So, those are all strange terms. Just to give you the flip side of this, here is a TH cell, it's an immature CD4+ cell. So, it has CD4 on it and that interacts with MHC II. When that happens, that up-regulates the TH cell and the TH cell runs around and turns everything else on. We're going to go through this later on slowly, carefully, piece by piece, but I thought it would be helpful to you to know that I did a quick table that puts these two cells sort of in a compare and contrast situation. That is, what is the difference between a TH or helper cell and a TC or in its maturity a cytotoxic T cell. The TH is here to coordinate responses, the cytotoxics are to kill. They both use the same kind of receptor which is highly specific for a little peptide. But when he comes to the co-receptor, they have two different co-receptors that each interact with its own class of MHC. In general, TH cells are alerted by specific antigen presenting cells and cytotoxic T cells can be alerted by pretty much anything with a nucleus in it except sperm. The sources of the antigen are different and their production is different and those are things we're going to get into later on. So, just use this as a sort of fast and dirty reminder of a whole bunch of terms in the immune system and especially reminder that they are constantly interacting with each other and are part of a feedback mechanism that up-regulates the immune response, and tailors it to the particular pathogen that provides a threat. So, one last tidbit before I sign off. That is the distinction that I made between adaptive and innate immunity was a little naive. That is, when we look at vertebrates compared to all other eukaryotic organisms, vertebrates are the only ones that change their DNA to make different types of antibodies and we'll see T-cell receptors. So, I use the distinction between change your DNA and don't change your DNA to distinguish between adaptive and innate. However, bacteria are capable of changing their DNA using a completely different mechanism involving an enzyme called crisper. What bacteria do is they actually copy the DNA of pathogens, use it to make RNA and then use that RNA to turn off the pathogens and it turns out that we can use this DNA for our own various purposes and it turned out to be a very useful tool that we now have in our kit of molecular biology. On the other hand, it's not remotely what we're doing here when we talk about adaptive immunity. So, just for fun I've given you the references for this bacterial system at the end of the outline and we're going to stop here. In the next lecture, we're going to develop the terminology that we use to describe the cells and systems and the immune system and then start talking about the specifics of innate and adaptive responses in the next series of lectures.
