Role of Phagocytes in Innate or Nonspecific Immunity

Role of phagocytes in innate or nonspecific immunity. Neutrophils, macrophages, and dendritic cells. MHC II.

The whole point of the immune
system is to keep out shady

things from your body-- or, if
they get in, to kill them.

So those shady things would
include shady proteins that

can do damage to your body,
viruses, bacteria, even

eukaryotic parasites,
and then even fungi.

So all sorts of things that if
they were to enter your body,

they would cause some
form of disease.

These are collectively
called pathogens.

So the whole point of the immune
system is, on a first

line of defense, keep these
things out-- and then if they

were to get into your body, to
kill and eliminate them from

our system so that we don't
get sick and so

that we don't die.

So I already just mentioned that
there's kind of two lines

of defense and even with
those, there's kind of

subclassifications.

The first line of defense-- I'll
just call that the first

line-- which is essentially just
to keep things out-- keep

all of these pathogens out.

And there's some obvious ones.

There's our skin.

Our skin keeps pathogens out and
actually even the oils on

our skin are a little bit more
acidic and it's hard for some

types of bacteria to thrive in
that type of environment.

You have your mucus membranes
and in the mucus, there's

there's some chemicals that
maybe make it a little bit

more difficult for bacteria
to survive.

And then you even have acidic
environments like

your stomach acid.

You might not view your stomach
as the outside of you,

but it fundamentally is.

Your whole digestive tract,
which I'll make videos on in

the near future, is really
on the outside of you.

You can simply model really most
vertebrate bodies as kind

of a doughnut our digestive
tract is the

inside of the doughnut.

So stomach acid is on the
outside of our real bodies and

you can imagine, that's a hard
environment for a lot of these

pathogens to survive in.

So that's the first line of
defense, but we know that

that's not good enough, that
sometimes these things can get

into our bodies, and there we
have to start thinking about

the second line of defense.

What do we do once things are
actually in our body?

And here, in both the first and
second line, I'm talking

about non-specific immunity--
and this is going to make a

lot of sense when we start
talking about specific

immunities.

So both of these are
non-specific.

And when I say non-specific--
or you can also call them

innate-- it means that they
just generally respond to

things that appear bad.

They don't remember the bad
things that came before.

They don't respond to a
particular type of virus or a

particular-- well, they do
respond to every type of virus

or every type of bacteria, but
they don't say, this is virus

type A, B, C, or this is
bacteria type A, B, C.

They just say, this
is a virus.

Let me get rid of it, or
let me not let it in.

This is a bacteria.

Let me get rid of it or
let me not let it in.

It doesn't know what type of
bacteria it's dealing with.

So this is all the
non-specific or

innate immune system.

And we'll go into a lot of
detail on the specific immune

system because you can imagine,
it becomes very

complicated or interesting when
you start thinking about

your body somehow remembering a
virus that it's seen before

and being able to respond better
to that virus or that

bacteria or that protein the
second time it sees it.

So we're dealing with
non-specific in this case.

And the the second line of your
non-specific immunity,

there are two things.

One is an inflammatory
response.

And I'm going to do a whole
video on this, but in general,

we've all experienced
inflammatory responses.

When you see blood flowing to a
certain part of an area and

you see there's pus and
there's-- and I'm going to go

into a lot more detail on what
an inflammatory response

actually is, but that's one of
your-- and what it really is

doing is bringing blood and
bringing cells that can fight

whatever type of infection you
have. It's bringing them to

the site where maybe you got a
cut or maybe where a lot of

the bacteria or whatever
the pathogen is.

So inflammatory response is all
about bringing fluid and

fighters to the fight.

I'm going to do a whole
video on that.

But the byproduct is, that part
of your tissue or that

part of you body gets inflamed--
a lot of fluid

there, a lot of byproducts of
the battle that goes on there.

We'll do a whole
video on that.

And the other second line of
defense is, and it's actually

part of the inflammatory
response-- are phagocytosis or

phagocytes.

And really, what I want to do
over the rest of this video is

talk in a little bit more
detail about phagocytes

because once we understand what
phagocytes do, that's a

pretty good building block for
going into the specific immune

system-- and actually, it'll
help lead into the discussion

on the inflammatory response as
well because phagocytes are

really part of the inflammatory
response.

So phagocytes are just
a class of cell

that can eat up pathogens.

They can eat up other things
really, but when we talk about

the immune system, we're talking
about pathogens.

So let's say that this is
a phagocyte right here.

This is a phagocyte
right there.

It has some kind of a
nucleus, whatever.

I don't have to focus on the
inside of the phagocyte.

It's a traditional eukaryotic
cell, but what I want to do is

see what happens when a
phagocyte encounters a foreign

particle or a foreign
bacteria.

So let me say this is a foreign
bacteria right here.

So the phagocyte, we've already
said, is non-specific.

What it does is, it has
receptors that respond to just

things that it knows are bad.

You could imagine these
are super sensors.

Maybe these are super sensors
for bacteria.

The bacteria have proteins on
their surface that maybe look

something like that.

Obviously they don't look
exactly like that.

I'm just drawing them as kind
of a Y and a triangle so you

can see that they fit.

But once these two guys
connect-- let me draw the

situation where they
have connected.

So this is the bacteria.

This is the pathogen.

And it's really the same idea
with a virus or any

other type of thing.

And we'll actually see in future
videos that these guys

can actually be tagged by other
molecules, which makes

these phagocytes want to
attack them even more.

Once they're bonded--
that's my bacteria,

the invading pathogen.

And now it is bonded.

It has triggered the receptor
on this phagocyte.

This phagocyte will start to
engulf-- it'll wrap around

this pathogen.

And these two ends are
eventually going to meet.

But then once these two meet,
what's it going to look like?

Then all of a sudden, that
bacteria is going to be

completely engulfed.

It's going to be inside
of the cell.

So now the cell-- once these
two ends meet and these

membranes merge, then this guy
is going to be in his own

little membrane bubble-- or you
can almost imagine, it's

in its own little vesicle.

So this is the pathogen, the
bacteria in this case-- but

phagocytosis-- the process is
completely identical in terms

of how it engulfs things.

If it was a virus or some type
of other foreign protein or

any type of really foreign
molecule-- actually, sometimes

it doesn't even occur
to foreign stuff.

It can occur to dying molecules
that are not

foreign, that just need
to be cleared out.

But we'll just focus on the
immune system, on foreign

things right now.

So this membrane right here will
completely merge and go

around this guy like this.

And of course, you had your
receptors and who knows if

they're still there.

By the time-- let's just draw
them there so you see that

that part is that part.

But once it's fully engulfed,
this thing is called a

phagosome, on which is really
just a vesicle that contains

that foreign particle that
you want to get rid of.

And then other fluid or vesicles
that contain things

that can eat up this phagosome--
so let's say that

this is some vesicle that
contains things-- lysozymes

and it contains really reactive
species of oxygen.

And if this comes in contact
with, really, almost any

biological compound, it's
going to do some damage.

But once the pathogen is
completely merged inside the

cell, this little package will
merge over here and it will

dump its contents into this
phagosome, into this vesicle

containing the pathogen,
and then break it up.

It's essentially digesting it.

So obviously the first role is,
it just got it out of the

way and it killed it.

And then the second role-- and
I'm just going to give a

little tidbit right here.

We're going to do it in a lot
more detail in future videos.

It breaks it up.

So now the thing is
all broken up.

So that thing is broken up into
constituent proteins and

another molecules.

And then what the phagocyte
does-- it'll actually take

some subset of these
molecules, some

subset of the proteins.

It'll break them out.

Proteins are just sequences
of amino acids.

Normally when people say
proteins, they're talking

about long sequences
of amino acids.

When people talk about short
sequences of amino acids or a

protein that's broken up a lot,
they refer to it as a

peptide chain.

A peptide chain is a shorter
chain of amino acids.

So this guy will take some
special peptide chains, some

special pieces from the thing it
just killed, attach them to

some other proteins.

So it'll take maybe a little
piece of this bacteria right

now, attach it to other protein,
which is called a

major histocompatibility
complex-- and if we're talking

about phagocytes, this will be
a major histocompatibility

complex type II.

It sounds very-- a strange word,
but we're going to see

this a lot.

So they abbreviate it MHC.

This is a protein and it bonds
with this peptide that was

kind of chunked off or digested
off of this invading

pathogen and then this phagocyte
will then present it

onto its membrane.

So this combination-- the
complex of the MHC-- in this

case, it's going to be
an MHC II protein.

We're going to talk about
Type II in the future.

It's going to take this complex
and then present it on

its surface.

And the reason why I'm going
through all this pain of

explaining this process-- you're
like, hey, we already

got rid of the thing
and killed it.

Why is Sal worried about what
we do with the peptides?

This is crucial to our immune
system because we'll see other

specific parts of our
immune system.

Remember, so far everything
is non-specific.

This guy just said, this
is an invader.

It doesn't know the type
of an invader.

It just says, hey, let me bond
to this thing and kill it.

It's one of these things that I
know are foreign to my body.

So it kills it, but now it can
leave it on its surface and

now the specific parts, the
parts that actually have

memory and attack specific
things, can say, gee, Mr.

Phagocyte, look, you've
killed something.

Let me see if I have some
specific reactions that can be

triggered by this thing that
you're presenting.

So, many phagocytes
are also called

antigen presenting cells.

And I'm going to go into more
detail on what exactly an

antigen is.

I called this thing
a pathogen.

An antigen is essentially-- you
can view it as a protein

or a peptide chain that will
trigger or that can be dealt

with within the immune system.

I'll be a little bit-- the
specific immune system.

And I'm going to be a little bit
more nuanced about it when

I talk-- I'll make a whole
video on antigens and

antibodies, but right now you
can just view it as a peptide

chain right there.

An antigen is just a protein
or part of a protein.

So this is presenting an antigen
on its surface that

can later be used
by other parts.

Now, the one thing that-- there
are many, many types of

phagocytes.

And just to give you-- just so
when you see different words,

you don't get confused by the
different types of phagocytes.

I'll do a little review
of those right now.

You have neutrophils.

These are actually the most
common of the phagocytes.

And these are the fast and
numerous respondors.

So these get to a location
of infection very fast.

Phagocytes don't necessarily
just have to kill in this way.

I mean, they're called
phagocytes because they engulf

this way, but we'll in future
videos talk about other ways

that they can release chemicals
or even DNA nets to

ensnare pathogens,
but neutrophils

are fast and abundant.

And then you have macrophages,
which are-- on some level,

they're the most versatile and
do the heavy lifting, but

they're also phagocytes.

And then you dendritic cells.

And when you first see the
word dendritic cell, you

think, hey, does this somehow
relate to dendrites of the

nervous system?

And no, they have nothing to
do with the nervous system.

The reason why they're called
dendritic cells is because

they look like they
have dendrites.

So they look like neurons on
some level, but they don't

participate in the nervous
system at all.

And these tend to be the best
activators of the specific

immune system that we'll talk
about in future videos.

So anyway, I'll leave you there
and we'll talk more

about all of this in the
next few videos.