The lymphatic system's role in immunity - Lymphatic System, Medical Video

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.
It would be hard to feel
something that's 1 millimeter.
And so let's look at
where some of these
are located on a human body.
Let's actually give this
guy a neck to be generous.
So the lymph nodes
are, obviously,
located all along lymph vessels.
But because there
are 600 of them,
they're located along the
larger lymphatic vessels that
have already collected many of
the little lymphatic vessels
and consolidated them.
So let's start with places
where you might actually
feel lymph nodes
on your own body.
So often, you can
feel the lymph nodes
that are in the
inguinal region there,
kind of between your
thigh and your abdomen.
And as we know, there are kind
of large lymphatic vessels
that consolidate here and
merge and rise up the body.
And all along here, you
might have lymph nodes,
all along there.
And then often, when a
doctor gives you a check up,
he'll feel for lymph nodes
along your clavicles there.
He might feel along your neck.
And he might actually feel on
your face and below your jaw.
So all those are along lymph
vessels that are coming down.
And then the lymph vessels
that are coming here
from your arms-- those will
have lymph nodes along them.
And so in this way, you have
about 600 of these lymph nodes
sprinkled about in your body.
And the cool fact to remember
is that any lymph that's
coming out of any
tissue in your body
will pass through at
least one lymph node
before it goes
back to your blood.
So even if it's out
in your little pinky
finger or something, the lymph
vessels that carry that back
will come and join a
larger lymph vessel,
which will pass
through a lymph node.
Or even if it's here in
your stomach, it'll come
and it'll join, and
it'll pass, and it'll
get into a larger one
with a lymph node.
So really, all the lymph that
goes back into your blood
is filtered.