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  • We've talked about how the purpose of the lymphatic system

  • is to collect fluid that's squeezed out of blood vessels

  • like this one-- to collect that fluid

  • and eventually bring it back into the blood

  • so that we don't lose too much fluid.

  • So this is a blood vessel.

  • Around it, we have some cells that

  • are maybe being fed by the blood vessel.

  • And at the same time, we have these lymphatic vessels

  • that kind of start out of nothing.

  • And they're porous so that they can collect the fluid that

  • was squeezed out of the blood vessels.

  • And then eventually, they'll put that fluid back into the blood.

  • But the lymphatic system actually has other purposes.

  • It helps us in other ways.

  • And we're going to talk about one

  • of those other ways in this video.

  • And to talk about that, we need to talk a little bit

  • about infection.

  • So infection is when your body gets

  • attacked by a foreign invader.

  • And that invader will usually be a bacteria or a virus.

  • And in this case, let's talk about a bacteria.

  • So let's say we get a bacteria that invades this tissue here.

  • So the first thing you might ask is,

  • why am I drawing the bacteria here?

  • Why am I drawing it outside of the blood?

  • Maybe I should be drawing it here inside the blood.

  • And one reason why I might think that, why you might think that,

  • is because think about when you get infected.

  • Sometimes, you get infected when you cut yourself, right?

  • When you cut yourself, you might get an infected wound.

  • And when you cut yourself, you see blood.

  • So you might think that you're getting

  • infected because bacteria or viruses

  • or whatever are getting into the blood.

  • But actually, that's not quite true.

  • Infections are usually not in the blood.

  • And actually, there's a pretty simple, you could say,

  • proof of that, which is that when you get infected,

  • infections almost always stay localized.

  • And by that, I mean that they usually stay in one place.

  • So think about when you get an infected cut.

  • Usually-- let's say, it's on your finger--

  • the infection will stay on your finger.

  • Or let's say you get a UTI, which

  • is a urinary tract infection.

  • The infection usually stays in your urinary tract.

  • Or maybe you get pneumonia.

  • The infection stays in your lungs.

  • So if the infection was actually in your blood,

  • it would immediately go all over the body.

  • And that does happen sometimes.

  • But it's way more serious than the majority of infections.

  • So really, the most accurate way to draw a typical infection

  • is to draw the infection occurring in the tissues.

  • Now, when we have this infection,

  • there are going to be some local immune cells that your body has

  • that's going to be fighting these bacteria

  • and trying to kill them.

  • But your body also has more powerful, really,

  • defenses, which consist of B cells and T cells.

  • And these guys are part of the adaptive immune system, which

  • means that they actually sort of react to the specific invader

  • that's there.

  • So unlike these guys here-- which, for the most part

  • are macrophages-- unlike these macrophages, who

  • can fight lots of different kinds of bacteria,

  • these B cells and T cells will react to the specific bacteria

  • and end up having a much more powerful effect against them

  • because of that.

  • They're specially tailored to the invader.

  • But the problem is that these guys don't just

  • sit out there in the tissues where the infections start.

  • And there are a few reasons for that.

  • But probably the most important is

  • that for these B and T cells to specialize

  • against these invaders requires a very complex process that

  • couldn't possibly occur just willy-nilly out

  • here in the tissues.

  • They require a special environment,

  • which we can draw here, which is kind of like a training camp

  • where they can develop in response

  • to this specific invader.

  • But now, we're faced with a problem,

  • because we've already said that these bacteria stay here

  • in the tissues.

  • They're not swept into the blood and then carried

  • to the B and T cells over there.

  • They stay in the tissues.

  • And these B and T cells are not out there in the tissues.

  • So how can we get these B and T cells to see these bacteria

  • and be able to react against them?

  • And that is precisely what this lymphatic system

  • does, because these bacteria are going to get swept in.

  • And at the same time, these macrophages

  • might gobble up some of these bacteria.

  • And once they've done that, they might

  • migrate into these lymphatic vessels.

  • And it turns out that the way your body

  • resolves all this is to sweep all these things directly

  • to the nearest lymph node.

  • And that's what this structure is called.

  • It's called a lymph node.

  • It's called a node because it's generally

  • a smallish object in your body, and it looks somewhat circular.

  • So it's like a node.

  • And the word lymph has to do with the fact

  • that we have B and T cells here.

  • And it's also convenient, because what

  • leads to it is the lymphatic vessel.

  • So these bacteria and some of these macrophages carrying

  • the bacteria will get swept along.

  • And they'll be in contact with these B and T

  • cells, who can then multiply and specialize and get

  • ready to fight this infection over here.

  • And we're not going to talk about how they ultimately

  • do that, because that's a topic, really, for another video.

  • But I just wanted to mention that, as we said,

  • the primary purpose, really, of the lymphatic system,

  • of these lymphatic vessels, is to carry fluid back

  • into the blood.

  • So obviously, by the time all this stuff

  • gets here to this lymph node, that

  • can't be the end of the lymph vessel.

  • So lymph vessel is actually going to continue.

  • It's going to continue, and it's going to eventually carry

  • fluid and such back into the blood.

  • So ultimately, the fluid is going

  • to end up back in the blood.

  • And so actually, another nice side effect

  • of having these lymph nodes here interspersed

  • throughout the lymphatic vessels is that they'll actually

  • filter all this fluid that's gonna

  • get put back into the blood.

  • If you didn't have this lymph node here,

  • you would still need the lymphatic vessels

  • to get rid of the fluid that your blood is filtering.

  • And what would happen is these bacteria

  • would get just put directly back into the bloodstream.

  • And that would be a problem, because as we said,

  • infections of the blood are way more

  • serious than local infections of tissues.

  • So this lymph node acts as a filter

  • for the lymph, which is passing through here.

  • And actually, it also has some macrophages here

  • that are going to help gobble up some of the bacteria that

  • might be floating through.

  • And so in sum, I think we're left with something

  • that's really a very elegant system.

  • What happens is we've developed a way

  • to sweep all these bacterial invaders basically

  • to the nearest police station.

  • And at the same time, we've developed a system

  • to filter the fluid that got squeezed out of these blood

  • vessels, to filter it as it passes through here,

  • so that by the time it gets to the end,

  • it's free of bacteria and other debris.

  • So before we finish, let's talk a little bit more

  • about these lymph nodes so we have

  • a sense of what they're like in reality.

  • So they're generally fairly small.

  • They're definitely much smaller than your average organ.

  • And so they're not considered organs.

  • And they're about 1 to 25 millimeters.

  • And they're small enough that you can have lots of them

  • in your body.

  • And so you do.

  • You have about 600.

  • There are about 600 in the body.

  • And some of them are located close enough to the skin

  • that you can feel them.

  • And those would obviously be the bigger ones.