Welcome to lecture two. Now, I did warn you that we had a lot of complicated and boring terminology, all of it with multiple syllables and Latin and Greek derivations. What I've tried to do in this lecture in giving you the overview to the tissues and organs, is to give you some tools for making the process of, sadly, memorization a little less onerous. So, if you get your outline out, at the end of your outline, you're going to see a table that says analogies. It'll be helpful if you just have that have your hand in there and that'll show you some ways of trying to remember what all of these stupid terms are. The other thing that I'm doing is, I'm going to start taking these things and I'm going to try to organize them in such a way that you can make sense out of them. So, the first thing I've got after this, is a division of labor in the immune system. So, here we have the difference between a primary organ and a secondary organ. A primary organ is not too surprisingly where things start out. This is where most of the initial cell division that forms these cells takes place and if we're looking at the adaptive immune system. The primary organs are where the genes rearrange to make all of these exquisitely varied bunches of recognition molecules, and we will of course look at that process in detail. A secondary organ is an organ where the immune cells are activated, counter-antigen, talk to each other. So, primary organs include the bone marrow for B cells and most will see myeloid cells, our next distinction and the thymus for T-cells. Secondary organs we will look at again, towards the end of this lecture, those are things like lymph nodes spleen and regions of the gut. So again, we have these divided into two categories according to their basic functions. Now, the other distinction I gave you again and again and just as a reminder, we are going to look at the difference between innate and adaptive cells. The principal difference between these is if they are adaptive, then they have to rearrange genes first and undergo selection. If their innate, they are there from the get-go. Now, the last distinction I have to make in this is a little bit tricky in some ways I find that my explanation sound kind of circular. But let's go look at it again. Myeloid cells are all going to be innate. The lymploid cells are the ones that are found mostly in the lymph and the myeloid cells will be found everywhere. But lymphoid cells are found in the lymph well gee thanks that's very helpful. Well, this is the way it works, when we look in the next section on the signals that cause cells to differentiate. We will see that one of the primary first decisions that a cell makes, as it turns into blood cells, is whether to become myeloid or lymploid. If it makes the decision to become lymploid, the cells that it produces will be primarily concentrated in the lymph and in the secondary lymploid organs at maturity. The lymploid cells are going to include your B cells and T cells. So, it turns out that everything that is adaptive, is also lymploid. There are however some lymploid cells that are not adaptive are considered to be innate. So, in addition to the B cells, the T cells we have NK cells which we'll look at and we'll look at all of these in detail later. We also have some of our what I would call sentinel dendritic cells and some of those will also be myeloid as well. So, we have these kinds of distinctions that I hope will help you to organize your 4A into the individual parts of these cells. I would also like to say I'm about to start showing you huge numbers of figures in the next part and I have all of those figures in the figure lists so you don't have to do screen captures. There you have it we're going to go into the details. There is one last thing I need to warn you about, and that is the term CD. I'll refer to this one as the term from hell. The actual CD stands for cluster of differentiation and that simply refers to how cells come out of various separation procedures that involve a technique called flow cytometry. So basically, what happens is you pop the cells in the top of this thing, you flow them one way, you flow them another way and you wind up with little clusters. Then you name them cluster of differentiation and that you give it a name one on up to- I don't know it seems like it's heading towards infinity. Okay, so what the word CD means is simply a tag that tells you the order in which it was discovered. Now, it doesn't tell you what it does, it doesn't tell you how big it is, it doesn't tell you what kind of molecule it is overall, it just tells you, and this is somewhat important, is that it's a molecule on the surface of some cell that influenced where it came out during the separation. You see these terms all the time. If you're likely to have heard of anyone, the one you're most likely to have heard of already is CD4. Because in HIV, the CD4 plus cells which are usually Th cells, are the cells that are attacked. What they're simply saying is that these cells have a protein on the surface that we have identified as the fourth one discovered and that protein which we'll see is uses a co-receptor, will be something that these viruses can attach to. So, the sad thing is that a CD28 molecule is not bigger than a CD4, it was simply discovered later. There are categories in these CDs that say which ones are receptors and which ones are signals. There aren't categories that tell you which ones have immunoglobulin regions and which ones do not. It's simply a first pass-through that tells you when it was found and then the people who found it will give some characteristics of it. So, what happens is that once something is labeled CD4, it sometimes gets another label as well and a common name. So, we will see that there will be places where I say this is CD whatever and it's also referred to for example CD5, is referred to as one of the subunits of the IL-2 receptor, and that can be incredibly confusing and I apologize for that. If I had been the boss emperors of the known universe I probably would have found a better system, but this is the one you're stuck with and I'm doing my best to help you deal with it. So, with that in mind, we're going to start into looking at the formation of all of these blood cells and I'm going to try to start using some additional techniques to make it a little more interesting and a little more memorable.