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Immune System

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The immune system protects the body from diseases. It detects and kills things like viruses and bacteria to prevent sickness and to allow the body to continue to function properly. There are three lines of defense which protect the body. The first line is eternal (skin, mucous, tears). The second line involves phagocytosis, which is the ingestion of invading organisms by white blood cells. The third type is specific immunity in which white blood cells recognize previously encountered organisms.

The immune system is the defense system of your body. It's what protects you against foreign invaders as well as cells of your own body that wind up going out of control for example cancer. Now you have series of non specific defenses that defend against the broad range of attackers versus specific defenses which are cells that will work against one and only one particular kind of attacker or disease. The non-specific defenses includes in the first line of defenses some barriers and chemicals. Examples of some these barriers would be like your skin. Your skin blocks pretty much anything that can't make its way through by going through a cut or something like that it blocks everything. It doesn't sit there and go you're tuberculosis you stay out but come on in measles no. It is a non-specific, you have chemicals such as the juices, digestive juices in your stomach, gastric juices that whenever you swallow any of the bacteria that might have been in your food or that you accidentally inhaled but got stuck in the mucus another non-specific defense, when you swallow those things they die in your stomach. Thus protecting you against the effects of those fungi and bacteria and such.

A second line of defense is once something has managed to get past those outer barriers, once they're in your blood supply or they're in some of your tissues how else can you defend against them? Well you got a whole bunch of cells called phagocytes which include things like monocyte which mature into the macrophages which you may have read about, neutrophyls and some other cells. And what they do is they go around and they just eat anything they don't recognize. Imagine you had some kind of psychotic group of kindergarteners that whenever they saw something they didn't recognize they would eat it. That's what they're doing, they help protect you against all sorts of things, you also have various chemicals such as interferon which is a chemical given off by a cell that's been infected by a virus. It helps alert the other cells around it so that they become more resistant to the virus.

Inflammation is another non-specific defense that is used by our body, if you get a cut or a splint or something it'll trigger off the release of various chemicals including something called hestomy which will open up the blood vessels in that area plus make those blood vessels more leaky. That brings more blood to allow the phagocytes and other white blood cells to get out of the blood stream and attack whatever it is that caused the injury. When you do need a specific defense, that's when these guys kick into play. Now they attack what are called non-self antigens. In general an antigen is anything that will trigger off an immune response. We will sometimes talk about HIV being an antigen or tuberculosis could be an antigen. There's actually many different kinds of molecules on the surface of HIV for example that act as antigens that your body detects as non-self i.e. they don't belong in your body.

Now every B-cell, every cytotoxic T-cell, every helper T-cell and even these things called suppressor T-cells they respond to one and only kind of antigen. So must have millions of these different kinds of B-cells to be able to attack the various kinds of antigens that you're going to be exposed over the course of your life. And this is accomplished through some interesting genetic tricks inside some portions of your DNA. Now the B-cells, these are defenders that make antibodies, antibodies are proteins that stick to foreign antigens don't get those two confused. It's very easy because they both begin with anti. But the antigens generates the response which includes the antibodies which are these little protein again that stick to form non-self antigens.

Because these proteins are dumped into the blood supply which is a fluid an order name for this is called humoral response because humoral is an order term that means fluid. Now when a B-cell gets activated and is allowed to go on the attack they mature into what is called a plasma cell. A plasma cell is an active B-cell and it is working so hard that it actually will wind up killing itself within not too very long maybe a week of two. And that's because it's pumping up thousands and thousands of antibody proteins per second. If you stop to think about that, that's insane, if you ever want to get the idea of the good analogy of what a plasma cell is like go on to YouTube and type in mini gun that's this gun that spits out something like six thousand rounds per minute just sitting there going, that's what these plasma cells are doing.

They're sitting back maybe in your lymph nods and they're just going, shooting out these antibodies to try to make them stick to the foreign invaders to target them from destruction plus antibodies can trigger all sorts of other problems for the foreign invaders. Now every time you get an infection and activate these specific defenses you'll also generate memory cells. Memory B-cells are unlike plasma cells they're not pumping out thousands of antibodies per second but instead they're just sitting there laying in reserve incase you ever meet that invader again. That's why if you get say chicken pox as a kid you'll never suffer from chicken pox again. Although sometimes if your immune system gets suppressed like if you're going through chemotherapy or something like that, sometimes you can have a re-infection or if you get stressed and stress can reduce the effects of your immune system.

You can sometimes develop a secondary infection called shingles. Cytotoxic T-cells instead of working at a distance like the B-cells these get up close and personal. Cyto means cell, toxic means of course poison. These cytotoxic T-cells I think of them sometimes like CLT members what they do is sit at range and attack with cruise missile antibodies instead they're sneaking up close to the bad guys and stabbing them to death. That's what they do, they actually latch on to the membranes of foreign or infected cells or cancer cells and they start pumping in proteins that burst open the membrane of their victim. Now they too will form memory cells after an infection is done and those memory cells will give you and active reserve that can quickly respond to any future infection.

The helper T-cells are like the generals of your army. They actually don't attack themselves but they're the one that give the go signal to the B-cells iin cytotoxic T-cells, so that they can go and attack. Additionally they helper T-cells send off lots of signals that will trigger the B-cells in cytotoxic T-cells to start rapidly doing cell division. So you wind up not having maybe 10 or 20 B-cells that work against that infection. But you'll have millions of them and that's one of the reasons why the lymph nodes where a lot of these cells are formed, that's why they swell up when you've got an infection. And that's why the doctor will sit there and poke it here going does that hurt?

If they're swollen that's because you're getting some kind of infection going on and you're creating this clone army against them. The helper T-cells they themselves will reproduce rapidly in response to their particular antigen. But they only do that when given the go signal or when they are given the antigen by a macrophage. Now the suppressor T-cells, they come along last. They're the once that turn off this whole process of it mounting a defense. They come along, they're kind of like the UN peace keepers, they come along and say come on guys let's stop fighting and your defenses go back down, and in fact when you get sick usually what makes you feel so grumpy is not the disease it's your immune system using all of your energy to create these millions of new cells and that's why we don't want to have millions of cells all the time. Yes they'll make you really well defended but you'll be wasting energy.

Alright let's take a closer look at how a helper T-cell gets activated. So again a macrophage goes around it anything it doesn't recognize in this case this red blob it eats and it goes around and puts those red blobs pieces of the red blob on its surface. Kind of like a police officer if he wandering around, he is a non-specific defender. He doesn't only arrest muggers or only arrest buglers, if he see somebody doing something that he doesn't think is right he arrests them. Then he takes them down to the station, now macrophage breaks them apart. Hopefully the cops aren't breaking somebody that he arrested. But he will take photos and then he'll start presenting the photos to various officers or detectives in the different divisions. He'll go to the bank division, he'll go to the kidnapping division and he'll find the one helper T-cell who is responsible for fighting that kind of criminal.

And when they dock together this helper T-cell starts to rapidly reproduce and then he finds the B-cells and activates them. The B-cells they may have already bumped into the antigen they go can I go, can I go, can I go but you don't want your B-cells to accidentally get activated just because you encountered one antigen because you don't want to have one virus particle get into you and your macrophage just deal with it quickly you'll only want to deal with it if there's an actual major invasion. So the B-cells have to wait until the helper T-cell comes along and says go they're given the go signal you wind up with gazillions of plasma cells pumping out their antibodies that's this little white shaped things sticking to the foreign invader.

The helper T-cells will also say hey macrophages you divide and so you get more macrophages and they go to the killer T-cells and say kill them and the killer T-cells says yeah and he goes and he starts using [per] that's the name of the protein that they use to stab to their victims. That's the specific immune defense, this is one of the things that I'm sure that you can get sick of studying because it's the immune system.