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Types of immune responses: Innate and adaptive, humoral vs. cell-mediated

The immune system is divided into nonspecific (innate) and specific (adaptive) responses. Nonspecific defenses, like skin and stomach acid, block pathogens indiscriminately. Specific defenses, such as B and T lymphocytes, adapt to specific pathogens after exposure. B lymphocytes are part of the humoral response, dealing with pathogens in body fluids.  Created by Sal Khan.

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Video transcript

In the last video, we talked a little bit about the immune system. In that video, we focused on the nonspecific or the innate immune system. So let me write that. And even in the nonspecific immune system, we subdivided that into kind of the first line barriers. And those were things like the skin, or the stomach acid, or the acidity of the oils on the outside of your skin. These are just natural barriers to not allowing things inside of your body, but then once they get in-- so you can almost imagine these were the first line of defense. And then you had your second line of defense, but these are still nonspecific-- and when we say nonspecific, it means that they don't necessarily know what type of virus, what type of protein, what type of bacteria. They just know that this thing looks shady. Let me eat it up. Let me kill it. Let me have an inflammatory response. So in there, we said, well, there's an inflammatory response, which I'm actually going to talk about after we do videos on the specific immune system. You have your inflammatory response, which really just gets things to where the action is at and then you also have your phagocytes, which are these cells that are engulfing things. And just so you know, all the phagocytes that we talked about in the last video, these are all instances of white blood cells or leukocytes. These phagocytes right here, these are all-- I talk about dendritic cells and macrophages and neutrophils. These were all white blood cells. These weren't all the kinds of white blood cells. We're about to talk about more-- and the other word for white blood cell is also leukocyte. So that is nonspecific. Well, one, it just doesn't let you in, but then when you're in, it says, hey, you're just shady. I'm going to eat you up. I have receptors. You have some double stranded DNA floating around. Only viruses have double strand DNA. I'm going to eat you up. I don't know what type of virus you are. I don't know if I've seen you before or not. That's why it's nonspecific. Now the really interesting thing about our immune system-- and this nonspecific, this exists across many, many, many species and types of organisms. But the specific is kind of a-- it's thought to be a newer adaptation. What I'm going to talk about is the specific immune system that's particular to humans. That's our other classification. Let me do it like that. So then you have your specific-- or you can imagine it's an adaptive immune system. You've probably heard of things like that. I have resistance to that bacteria or that virus. So this is adaptive. And it's really all based on having exposure to things. And within the specific immune system, we talked a little-- when we talked about the antigen presenting molecules that phagocytes do-- that plays a role in this. We're going to go into more detail, but I don't want to confuse you. But the main actors here are called lymphocytes, not to be confused with leukocytes-- because they still are leukocytes. So let me write this down. These are specific. Phagocytes, for the most part, are nonspecific, but both of these are white blood cells. Lymphocytes are another type of white blood cell or leukocyte. Don't want to confuse you with this convoluted diagram, but I just want to make the terminology clear. When someone talks about a white blood cell, they're really just talking about a set of cells that when people first tried to separate the components of blood-- you'd have your red blood cells that would kind of settle in the bottom, then you'd have this layer of white frothy stuff in the middle that was really made of white blood cells, and then on the top, you had the fluid, the plasma from your blood, kind of the watery part. So that's where the name came from, but they have different roles, but they interact with each other. Now lymphocytes can be divided into B lymphocytes, usually referred to as B cells-- and T lymphocytes. And the B and T just come from where they're developed. B lymphocytes were first recognized in the bursa of Fabricius. That's why it's called B. That's actually a part of birds that participate in the immune system. And so the B came from bursa, but B also applies to the human immune system because it's produced in bone marrow. So that might be an easier way to remember. It's produced in bone marrow. It's developed in bone marrow, but historically, the B came from the bursa of Fabricius, just in case you want to know. But it's easy to remember. The B could also stand for bone marrow because that's where it's produced. T lymphocytes actually do start off in the bone marrow, but they mature and become what they are in the thymus. So that's where the T comes from. Now in this video, I'm going to focus just on the B lymphocytes-- because frankly, if I focused on everything, it would be an hour-long video. But the B lymphocytes frankly on some level-- well, I don't want to pick and choose favorites, but something in my brain-- I just really like the B lymphocytes. So the B lymphocytes participate in what's called the humoral response. And I'll tell you what humoral means in a second. You'll see that T lymphocytes participate in what's called the cell mediated response and we're going to do that in a future video. They actually do certain classes of T lymphocytes. We'll see that there are helper T cells and there are cytotoxic T cells. I know it's all very confusing the first time you see it, but that's why I just want to focus on just this part right here. We're going to see in the future that the helper T cells play a role in amplifying and really activating this humoral response. But a simple way to think about the difference between the humoral response and the cell mediated response is, when I get infected-- let's say I get infected by a virus, right? At first, when a virus comes into my system, it's just floating around in the fluids in my system. The fluids of our system-- that's really what humoral responds to, into the humoral fluids of your body. So you have your viruses. These are little viruses floating around. So while they're floating around and they're not sitting inside of cells, that's where the humoral response can come into play. Same thing if we have little bacteria floating around and they haven't infiltrated cells yet. They're just floating around in the fluid, then the humoral response can be useful for that. Now if all of a sudden, these guys have infiltrated cells-- so if the cells are now infected with the virus and they're producing the viruses using the mechanisms of the cell to produce more, then all of a sudden we have to be a little bit more sophisticated in how we deal with these cells and how we deal with the viruses because they're not just going to be floating around anymore. We probably want to just kill this cell even though it was one of our own, but now it's helping to make viruses. Or maybe it's been colonized by bacteria. So in either case, you want to kill this. And we'll talk more about that in the cell mediating.