Foreign invaders that we'll call pathogens. These can be things like bacteria, viruses, or other microorganisms like single-celled eukaryotes, for example. The thing they all have in common is that they cause disease. That's kind of what defines a pathogen. It's something that's bad for us. I mean, really, you can just think of this as germs. They're germs. Now, the immune system is the defense network against pathogens that animals have, and it is comprised of lots of different cells and various tissues, and it is actually incredibly complicated and sophisticated. Here, we're really just trying to get an introduction. So there are 2 main types of immunity that we are going to talk about, innate immunity and adaptive immunity. Innate immunity is going to be able to mount the fastest response, and that's because these are non-specific defenses. And, the responses, as I said, are rather generic, so they're not going to be specific to the exact type of pathogen. That's the job for the adaptive immune system. The innate immunity is really just like your first line of defense. Adaptive immune system is an acquired specific defense that will be mounted against specific pathogens. And what's really cool is this actually confers long-term immunity. That's why, for example, once you get a certain virus, that virus won't get you sick again, even if, you know, it gets into your body because you have this long-term immunity against it, thanks to the adaptive immune, or the adaptive aspect of the immune system.
Now, the reason we need an immune system is because there are germs everywhere. There's germs on everything. Our bodies are constantly being assaulted by germs. It is an unrelenting onslaught. In fact, H.G. Wells basically said that we've earned our place in the natural world by constantly battling against pathogens. Here, you can see a nice microscopic photograph of red blood cells and some white blood cells mixed in there. Remember, there will be white blood cells moving around in the blood, just a very small percentage. Of course, that percentage is going to increase when you are fighting and actively fighting an infection. Now, here, we are looking at a cell from the innate immune system, and it is basically munching down these bacteria. The innate immune system cell is in yellow, the bacteria is in orange, and you can see that the yellow immune cell is consuming it. This is actually a process known as phagocytosis, and this is going to be a common tactic employed by cells of the immune system to deal with these foreign invaders, these germs. Phagocytosis, it's kind of like cell eating, but I don't really want to call it eating because it, you know, gives the wrong impression. This isn't necessarily how cells are trying to obtain nutrition, which is the purpose of eating. Right? Some cells will do phagocytosis to obtain nutrition. In our immune system, that's not the goal. The goal is to basically, break down the particle, as you can see happening here, and to create some chunks of the pathogen that can be recognized by the adaptive immune system. We'll talk more about that in a second. So with phagocytosis, the important thing I want you to note is that, yes, it happens with the cell engulfing a particle. As you see, the cell's like reaching out its membrane and wrapping this pathogen right here. But the reason it's doing that is because there is a response between ligand binding and receptors on the surface of the cell performing phagocytosis. So this is a ligand-receptor interaction that leads to this response. And that's what I really want you to take note of, that it is being carried out by a signal that's being passed from ligand to receptors. And yes, our immune cells will have receptors that can detect ligands on the surface of pathogens. And you'll see that that is going to, actually be an idea that we evolve later.
Now another thing that cells of the immune system are going to do is release cytokines. These are signaling molecules that help recruit other immune cells to help mount an immune response. And it's sort of like, putting a lure somewhere in the body for these immune cells to come. So whereas, for example, hormones signal by being spread out into the body, immune cells will come, release cytokines in a specific area where there's an infection, and this will cause other immune cells to go to that area and to mount an immune response there. So it recruits immune cells by attracting them like a lure to a specific place. Now, the real magic of the immune system happens with antigen presentation. This is going to be the thing that makes adaptive immunity such an amazing process. So what's going on there? Well, basically immune cells will present pieces of pathogens' membrane that contain recognizable proteins. That is, stuff that another immune cell is able to look at, recognize, and mount a specific response for. So what these cells are really presenting are called antigens. This is any molecule really that induces an immune response. Our adaptive immune system is going to build antibodies to those antigens. These are basically molecules produced by the immune system that recognize those specific antigens. And this is how our adaptive immune system learns to recognize specific pathogens and mount specific defenses. So here you can see some examples of antigens. Right? They come in different shapes, and colors, and sizes, and antibodies are going to be specific to an antigen. So you can see here at our antigen binding site, right here, it is of a shape that is specific to binding this particular antigen. Right? So antibodies are going to be specific to antigens. And behind me, is an example of antigen presentation. We have an antigen-presenting cell. It has an antigen here, well now it's a blue dot, this is the antigen, and it's going to present it on its surface to an immune system cell, in this case, we're looking at a T cell, and this T cell is going to be able to recognize that antigen, or take that antigen, and create a specific response for it. So this is just sort of a general idea, a general overview of antigen presentation. When we talk about the adaptive immune system, we will go into much more detail. I just want you to get the general sense of how our immune cells deal with stuff. Right? There's phagocytosis, there's literally consuming it, bringing it into a cell, getting it out of an area where it can do harm, and there's signaling to other immune cells, but most importantly is this antigen presentation thing. Right? Presenting recognizable components of pathogens to cells that can create specific responses to those antigens. With that, let's flip the page.
