Welcome to lecture three. This is lecture three on innate immunity, and you need to get out your outline and any extra picture files that go with the lecture. And that will help keep down the amount of writing you have to do. So, in lecture three, we're going to discuss innate immunity. This is the immunity that is not a result of changes in the DNA, which we'll see is a very unusual way to make any kind of molecule. This is the immunity that is basically found throughout the living world. I mean, all creatures have some innate immunity. In particular, insects have a very interesting form of innate immunity, in which, while they don't change the genes, they have a gene that they have 38,000 different ways to splice. And they use that gene again to recognize self and non-self. So, insects have innate immunity. But, to make these things, they don't change their DNA, so it's still considered to be innate. Now, let's see, plants have even more different kinds of innate immunity than we do. We'll see that plants have a variety of pattern recognition receptors, of defensive peptides, of all kinds of nasty molecules that they will produce when they are threatened. Again, a plant is in a situation where it can't run and it can't hide, and so it's only defenses are to do a form of chemical warfare. I mentioned before that the adaptive immune responses was a feature of the vertebrate organisms only. Now, the vertebrates are members of the Phylum Chordata. The Phylum Chordata is mostly vertebrates, but it has a few rather primitive, we'll say, or at least stem organisms that are similar to those that probably gave rise to the more complex organisms that have jaws and backbones and skulls, and as it turns out, adaptive immunity. In this picture of a tunicate, this is an adult tunicate, it's sometimes called the sea squirt. While it is a member of our Phylum, it does not have adaptive immunity. The next one is a diagram of something called the Lancelet or an amphioxus. And while this thing looks tadpoley, it is not a tadpole, it does not have a backbone and it doesn't have a skull. All right. These are two organisms that have, what we call a notochord, they have a spinal cord that runs down their back, that's a dorsal spinal cord, even though it doesn't have vertebrae around it, and these organisms are considered primitive or early or atleast representatives of what used to be primitive or early members of our Phylum. They, of course, have innate immunity but they don't have adaptive immunity. Innate immunity is something you basically find everywhere. Adaptive immunity is something you only find in vertebrate organisms: that is fish, amphibians, reptiles, birds and mammals, and we've seen that before. Let's look at what do I mean by innate? Well, innate can encompass all things. One of the things that innate refers to are simply things like anatomical barriers. So that, your skin is an exquisitely designed immune system, it has in it, not only a stratified, quantified layer at the outside compose the dead cells, and that gives you a great deal of protection. Very few organisms can penetrate intact skin. That is to say, it takes a mosquito bite or an abrasion or a cut or a puncture or something along those lines to get through your skin. It's only like one parasite in the whole world that can make it through on it's own. Your skin also does you great favors, in that you can see with the gland cells here, it secretes a variety of compounds onto the surface. Now, there are sweat glands but there are also oil glands, and in some of these oil glands, you also secrete various kinds of peptides and other defensive molecules that help to keep down bad bacteria, and in many cases help or leave alone bacteria that help defend you from the bad bacteria. When we go into your lungs and your gut, these guys are maybe not quite so tough, but they also have mucus, they have enzymes like lysozyme, that can chew up bacterial coats. These are in addition to that, you have in your stomach acids, so that your stomach secretes not only enzymes and mucus, but it makes for an environment that's highly acidic and that tends to kill bacteria. Now, think about it for a minute. What things eat the most disgusting stuff? Right the first time, here is vultures. Vultures have in their stomach secretions, some of the most acidic stomach contents of any organisms out there, because let's face it, they are eating things that are already very ripe and have lots and lots of bacteria in them. We have this set of molecules that's essentially barrier molecules, not only constructing barriers, but in fortifying them with extra chemicals, and sometimes it's just the simplest hydrochloric acid, sometimes it's as complex as proteins and peptides that select for a particular bacteria. That's one form of defense. The other form of defense that we're going to look at next is something called inflammation and I'm here to tell you the inflammation is not an unmixed horror. That's the next section.