My name is Ed Hope, a junior doctor in the UK

And on this channel, we talk about medicine, the human body, hospitals,

all types of medical things and explain it in simple terms.

Over the last couple weeks, I've been looking at "Cells At Work"

I probably would have binge watched the whole season by now.

But I've been holding it back because you guys have been really

enjoying the review so thank you so much

So, let's crack on with episode 3 of "Cells At Work"

[Audio from episode clip]

So I recognise this chap

by his uniform.

Uh, this is one of the cytotoxic T cells

So you know the soldiers we saw in the early episode.

So, before they become these soldiers

they have this kind of immature state: why this chap has a cap on

that says "NAIVE".

So the job of these cells is actually to kill your own cells

when they either become infected or damaged.

[Audio from episode clip]

Hang on a minute, did the-[laughing]

did the white blood cell's shoe just fall off there?

I'm not quite sure what that's a reference to.

Cool, so this is the zombie episode. I've been looking forward to this one.

Usually at the start of the scene they tell you what organ you're in.

I'm guessing by the fact that we've got these shots of the sewers

and that we got an immature T cell here

that we're in the lymphatic system of the body.

Now, before med school, the lymphatic system

used to confuse the hell out of me.

And having it as a sewer is a really good analogy.

So you imagine all that fluid and nutrients that leak out of the blood vessels

to supply your cells. Well, how does that all get collected?

Well, it gets collected in a series of ducts

called the "lymphatic system", and they drain all the way

back up and go back into the bloodstream just above your heart.

Because it's a drain-like system, so any bacteria

and viruses will also drain into there,

so it's a really good place for our immune system to do its work of

recognising the bacteria and learning how to fight it.

That's why when we get a throat infection

you'll feel the glands - the lymph nodes - in your neck swelling up.

[#1, 2, 3, 4. We are cells at work.#]

[Audio from episode clip]

So, as we've predicted in the very first episode,

the zombies represent cells infected by a virus.

Another really good analogy.

I remember my highschool biology teacher asking me:

"Are viruses alive or dead?"

It was a bit of a trick question

because they're neither, because they're never alive in the first place.

The only way they can kind of reproduce and grow,

some of the things that a living needs to do, is through a host cell.

So just like a computer virus on a USB stick

it won't just replicate on its own, it needs a computer

to replicate that virus.

And it's really important to know that bacteria are different to viruses

'cause that's why antibiotics only work on bacterial infections,

not viral infections, even though the symptoms may be very similar.

[Audio from episode clip]

[Laughing]

[Audio from episode clip]

Wow! Respect to the macrophage!

[Audio from episode clip]

So, I was waiting for them to show this:

You may have noticed throughout the series that whenever a neutrophil destroys

a bacteria or a virus, that's it.

It just moves on to the next one, whereas the macrophage here

is capturing part of the virus so we can identify

the virus more easily later and therefore destroy it more easily.

The part that it's capturing, we call an "antigen"

and for that reason the macrophage is an antigen presenting cell;

because it will present this antigen to other immune cells

and activate them.

For example: it can activate the T cells that we've seen here

to turn a naive and immature T cell into a mature cytotoxic T cell.

The macrophage is also part of the myeloid family tree that I showed you in the last episode.

So let me add it to the gang.

So, remember we had this process called "hematopoiesis" - literally meaning: "production of blood"

that happens in the bone marrow. So we have our stem cell,

our myeloid progenitor cell, of which we can get our red blood cell,

our neutrophil,

and our megakaryocyte - the ones that produce platelets.

Also produced from this sort of family is the macrophage.

[Audio from episode clip]

This is pretty much what it's like when you first start life as a junior doctor.

[Audio from episode clip]

Quite a subtle one here, but seeing the neutrophils sort of migrate

between the buildings here is a nice little reference

probably driven by chemical messengers

to see what the immune activity's all about.

But we call this process "extravasation",

so they're able to sort of squeeze through in-between cells to get to where they need to go.

[Audio from episode clip]

So we see the infection is spreading quite rapidly.

We use the term "incubation period"

to describe the amount of time between the initial infection

and when symptoms are showing.

So this can be anywhere from a couple of days, like influenza - the flu virus we see here,

to many years in a virus like HIV.

So there's been so much going on in this episode already,

I wanted to just explain a couple of concepts that I keep going over.

So, the way viruses infect cells,

and also how we're able to recognise cells that have been infected by a virus.

[#Music#]

So here is our cell membrane

made up of sort of fatty molecules, that's why I've made it yellow.

And we have a nucleus.

So our nucleus contains all the genetic material for the whole body,

but obviously we only want certain ones for this cell to work.

This is our DNA all coiled up like so.

So let's just say for argument's sake that this is a cell in the throat;

so one of it's jobs is to produce mucus

that we saw in the first episode,

helps capture bacteria and viruses so we can get rid of them.

So let's just say that this cell wants to produce some more machinery

that helps produce mucus.

So how does this do this?

So just part of the DNA that we want will be turned into messenger RNA (mRNA)

through something we call the "transcription".

It's called messenger RNA because it then travels out into the cell

to the ribosomes.

So these are gonna represent our ribosomes.

So the messenger RNA will travel out of the nucleus

to the ribosomes

where it will turn the message into proteins, what we call "translations".

So in our example these proteins will make a little

biological machinery that will create some mucus.

So it's important to know that the cell knows what and when to do things

by communicating with other cells either directly or through chemicals

that interact with these receptors on it's surface.

So these receptors here are how the cell communicates.

So how to viruses take advantage of this?

So this is gonna represent our virus. So kind of in it's simplest form, a virus is

this protein capsule, so very different to our lipid membrane

that has a piece of genetic material in it.

In the case of influenza we have some RNA in.

The virus also has some receptors on it's surface.

Now, because we talked earlier, the antigens

are anything that our body can use to recognise a virus,

these are often called "surface antigens"

and you can see these surface antigens

sticking out on top of the virus in Cells At Work.

So, one of these surface antigens may happen to interact

with one of the receptors on the end of the cell

granting the virus access to the cell.

So that's why viruses only infect certain cells

so it may just infect a particular species,

like a virus that infects dogs might not affect us

or specific cells in a species so virus that infects our liver might not affect our skin.

In this example, we're using influenza so we know

it's got the right receptor to make this handshake

this kind of lock and key that can, uh, infect

the cells of our respiratory tract.

And you may say "Well why do our cells have these receptors if they can be abused like this?"

And it's because our cells need to communicate with each other, right?

Lots of complex interactions going on.

It's sort of like saying

"Well why don't you stop your house getting burgled by not having any windows or any doors?"

Well, it can't do the job of a house then, can it?

So, once inside the cell, the hijack really starts taking place.

So the viral genetic material enter the nucleus

and hijack our system here.

So the mRNA that goes to the ribosomes will be

information to produce more viruses...

[#Music#]

...that'll then burst out

of the cell and infect more cells.

[#Music#]

So how does your body know that this cell has been infected

and know it wants to kill it with the cytotoxic T cells?

Well first of all, we saw briefly

that the macrophages will break down these viruses and find out these antigens,

and so the cytotoxic T cells will be primed to know to look out for these antigens.

But currently it can't see any of these antigens on the cell.

Well your cell is very clever

so it's constantly putting anything from inside the cell onto it's surface.

So, as soon as this virus comes in here

your cell will put part of the virus

on the outside of itself.

And that can be anything, it could be the surface antigen,

it could be part of the DNA of the virus,

it could be part of the virus' capsule.

So now your cell is holding up a flag for the cytotoxic T cells

by putting parts of the virus on the outside of the cell

[Audio from episode clip]

Awww. I think we've all been there haven't we,

I mean, particularly taking me back to my medical career at med school

...this was... You never feel like you're quite good enough do you.

[Audio from episode clip]

So we go on this hero journey of discovery with Naive cytotoxic T cell,

meeting the dendritic cell,

we're getting spoiled for choice with our cells today.

So the dendritic cell is also an antigen presenting cell,

but it has a much more, kind of powerful, role in stimulating other cells.

Again, it's part of our myeloid family, so it would be rude not to add it to our family tree.

So here we have our dendritic cell joining it's cousins.

There is actually two types of dendritic cell, one that's produced from another route,

but we're keeping it simple today!

And it's called "dendritic" because dendritic means "tree-like"

so it has all these tree-like projections

that helps increase it's surface area

so it can fit more antigens on it's surface and

present more of them to the other immune cells.

[Audio from episode clip]

So it's a nice little story here, showing the maturation of the Naive T cell

after it sees an antigen from the dendritic cell

in reality this would happen slightly differently:

So if you imagine, all these Naive T cells would have slightly different receptors on them,

so it's only the T cell that binds strongly to the antigen on the dendritic cell

that will get activated and then proliferate.

Kind of like the glass slipper in Cinderella,

it has to match perfectly for that T cell to become mature;

and that's exactly what you want isn't it?

Because you want the T cell that can recognise that antigen the best

to be the one that matures and then proliferates

because that's the one can recognise the infected cells the best

and then kill those cells.

[Audio from episode clip]

Awww. So, if before his uncertainty was kind of like being a medical student,

this is how it feels when you graduate