Modern medicine has allowed us to really manipulate our technology to mount better defenses against pathogens. We basically have found ways to manipulate our own immune systems to save us the trouble of getting sick. We call this immunization, which is basically when an organism's immune system gains specific defense against some pathogen. This usually happens through what's called vaccination. Vaccination is going to be the introduction of a vaccine to prime the immune system to deal with later infections. Basically, it allows you to mount a primary immune response to some weakened or killed form of pathogen that's not actually going to get you sick so that if you ever encounter, in the wild so to speak, that nasty bug in its form where it can actually get you sick, you have a secondary immune response ready to go to beat it down. Now, a vaccine is a biological preparation that contains antigens and as I said, usually it's like a weakened or killed form of a pathogen. The name comes from the Latin word for cow, which is 'vacca', and essentially, if you're wondering why this has anything to do with cows, it's because the first famous case of vaccination in Western medicine, anyway, was with this guy right here. His name is Edward Jenner and he used the cowpox virus to vaccinate people against smallpox which was a rampant and deadly disease. And it was pretty gnarly how he did it. I mean, he literally took basically a two-pronged needle that looked like that and dipped it in a vial of cowpox so that a liquid droplet would kind of stick between the two prongs and then he would just prick you with it and make sure that some of that liquid gets into your blood. Pretty gnarly stuff but it worked. And this would actually be an example of a type of vaccine that contained what's called an attenuated virus. Basically, a virus that's cultured in a species other than the host so it won't infect the host cells. This is not exactly an attenuated virus because, you know, cowpox is its own disease but it's highly related to smallpox. It's just that it infects cows instead of humans so, very similar idea to an attenuated virus. You can also see vaccines that are called subunit vaccines that contain isolated viral proteins so basically, some scientist will take a virus and break it down and find some good antigens to use and then, inject you with that so that you can recognize the antigens but you never actually have to have the virus inside you. You can also get an inactivated virus which will be like a damaged virus that contains antigens but can't cause infection. Now, often in biotechnology, you'll see things called monoclonal antibodies used and these are just antibodies that are prepared from a single clone of B cells, specific for some epitope. And these will be used often in research, for many different purposes, but you probably will see that name come up. Now, you'll know what it is. Now, things can go wrong with the immune system. Allergies are a common example of an abnormal immune response to an antigen which we'll call an allergen. Basically, it's not something that's actually going to get you sick but your body still mounts an immune response to it. That's why, for example, if you're allergic to cats or something, the cat dander will make you sneezy. You can get that inflammatory response. Right? You'll get all swollen and puffy. It's just your immune system going a little overboard. Right? Identifying something as a threat that's not actually a threat. Now, allergies are unpleasant but they're not nearly as dangerous as autoimmunity, which is when an immune response is directed at self molecules and the cells of the organism. Essentially, the cells of your immune system attack and kill your own body. For example, in diabetes mellitus, it can be caused by an autoimmune reaction that has the immune system specifically target and kill the cells that produce insulin. And that's why, people with that condition can't produce insulin even though they have a pancreas because their immune system went haywire and just killed those insulin-producing cells in the pancreas. Now, HIV or human immunodeficiency virus is a very heavily researched virus that infects and kills CD4+ T cells and macrophages and this is obviously very dangerous because it's a virus that actually targets and attacks the adaptive immune system which is really bad news because it essentially can prevent your immune system from being able to mount an immune response not only against it but other pathogens as well. Those helper T cells, which are the CD4+ T cells, are needed for both the humoral response and the cell-mediated response. Remember, they're going to help activate cytotoxic T cells and they're going to help activate B cells to produce antibodies. So, basically, this virus attacks, one of the most vulnerable cells of the immune system if you want to just take the whole system out. Now, there's a condition that develops from HIV that we know as AIDS or Acquired Immune Deficiency Syndrome. Basically, this is it's not a separate disease. It's a severe weakening of the immune system from an HIV infection. If you have an HIV infection, for a long time or it's untreated for a while or something, it can lead to your immune system getting so weak that you can't fight off any infections. Any little thing can get you sick to the point where it can kill you and, that is sort of how HIV is a lethal disease is it just takes out your immune system so that even like a little cold or something could be fatal. Here, not to end on too dark a note, but here you can see a cell being attacked by HIV virus. So, with that, that's all I have for this lesson. Stay healthy guys. But if you get sick, now you know what your body's doing to fight against it.