We're looking a little bit more on developing our metaphors of how politics and defense are very similar in their function to the way the immune system works. I can often think of immunology as development in a foreign language. That is, we're looking at the development of defenses and we're using a set of terminologies that are very different from the ordinary ones used in development, and yet many of the mechanisms are very similar. But just to remind you right now about what's going on, I'm going to draw some parallels between the immune defenses in your body and the defenses of the nation. In both cases, they're very expensive. If you'll recall, I started out by talking about ATP. They're expensive. You need to develop them and use them frugally. That is to say, you don't want to waste a lot of money. On the other hand, if you don't have enough effort put into it, you could wind up dead. Where a country can wind up overrun by Nazis or what have you. So, it's a very necessary set of defenses. So, that the national defense of the country and the defense of your body are in some ways very similar and have similar aspects to them, we have different levels of engagement and different levels defense. Again, I'd like to remind you that the defense that you run against pathogens has very many interesting parallels to the kinds of defenses that you use against enemies when you're defending your own country. There are parallels in the sense that, they are both expensive. That is, the cost of a hammer to the army is often something that is printed in the newspapers. But you know, there's no cheap way to make an F15. Indeed, there's no cheap way to make some of the immune responses that we have. So, they are expensive and they are necessary. If you don't defend your body against the pathogens that are trying to take it over, you could very well wind up dead and the same is true for your country. The third one however, the fact that we must be frugal and that we must be careful in how we spend our resources in defending our body, also has parallels in national defense. So, I'm going to sort of outline three different kinds of overall approaches that we might take if we discover a bacterium or some other cell in the body. The first one is, a TH1 response. This is equivalent to all out war. That is, you're going to be raising a serious defense as you possibly can against this pathogen. If that defense damages some of your own tissues, if it demands a great deal of energy, if it makes you feel sick as a dog which means that it's difficult for you to do some of your normal functions, that's very similar to an all out real war. In this case, then, this is the strongest response you can make and you frequently make it to things like viruses and bacteria and other very serious pathogens. Your other response is, a TH2 response, which is more of a containment response. In the case of a TH2 response, are friendly response. It's the equivalent of diplomatic sanctions, trade embargoes, setting up all kinds of barriers between you and another country, but it's not as hostile a response and in general, it doesn't result in the damage to your own tissue that a TH1 response would. The third response is, a T-regulatory response. That's kind of a peaceful coexistence response. In this case, you are not going to attack the particular organism. You do this for a lot of bacteria on your skin, that are actually beneficial and help fight off other bacteria that are far worse. So, when your body meets a new antigen, a new evidence of a new organism, it has to decide on which of these three responses it's going to make. Now, the other thing is the parallel to military and army and policing and functions, this is, sometimes we make mistakes. That is, there are times when we have misdirected defense attempts. You can make a TH1 response to something like your own tissues, that would be an autoimmune response. If you make a TH2 response to something you need a TH1 response to, you can die. One of the things that we find with cancer cells, is that sometimes the cancer cells can trigger a T regulatory response, so that your body will leave them alone. So, in the next part, I'll will show you some different examples of misdirected responses that we will look at later at the end of the course, when we're looking at clinical applications of the immune system.