I wanted to start with a little history. These are some beautiful illustrations from the

Florentine Codex which is thousands of pages long. It was created over about 45 years by

this guy, this friar. And basically what we did was he studied Aztec culture and how it

changed over his lifetime. So we've converted a lot of this to English and you can find

some if it on Wikipedia. It's just fascinating. So you could see these rituals and these chieftains.

And he would describe all of it. But a sad part of that is that he describes how small

pox came and devastated the Aztec. And it's one of the reasons why the Spanish were able

to conquer them. And so they didn't have a immunity to it. And that's what this is about.

So basically what happens with any kind of a infection, a viral infection like this,

is you have a virus. And that virus is essentially going to gain entry inside your cells. It's

going to use the machinery of your cell to make copies of itself. And more copies of

itself. And more copies of itself. And eventually it's going to destroy the cell and it's going

to spread off to infect other cells. And so that seems awful. In other words when you

get a cold what's happening is that your cells are making viruses which are spreading to

more cells. And if it weren't for our immune system we would be devastated by all the viruses

that are around us. And so we have to have protection. And so I like the analogy of a

castle. A castle needs protection as well. And so if you're building a castle, you have

to think about a few ways to defend yourself. First of all you have to defend yourself from

the outside. So it's good to have a moat. It's also good to have a really large wall.

So it's hard for people to get over it. And this castle's great because it's got water

on all sides of it. You also have to have soldiers. So if anybody were to ever get close

to the gate you could shoot them with arrows or pour hot tar on them. But if they were

to breach the wall you have to be able to fight them there as well. But probably when

you're defending a castle a more important thing is you have to have intelligence. You

have to have spies that are sent out to surrounding areas to do reconnoissance and figure out

what's going on. To recognize invaders when then come. And so I'm going to quit talking

about castles and we're going to talk about the immune system. But the same thing works

inside us. And so the idea of a castle wall. Let's start with that. And so what protects

us from infection, our castle wall is going to be our skin. And so what our skin provides

us with is it provides us with a barrier. So there's going to be a barrier of cell,

dead cells, on the top. And keratin on the top. It's also going to have a really low

pH which makes it hard for any kind of bacteria to live there. And we're also going to have

chemicals on the surface of our skin that are going to disrupt certain viruses. And

also we're going to have bacteria that crowd out our skin. And so it makes it hard for

other bacteria to gain entry. We have what are called normal flora that just live on

our skin. And so all of this is going to provide protection against infection. But occasionally

you know that that get's breached. Occasionally you cut yourself or a pin prick or something

like that gains entry. And so that would be just like the soldiers making it over the

wall. And so what do we have? Well we have a call to arms. We have inflammation. So basically

we have chemicals that are released that cause our body to respond to that. Now you constantly

are being infected, especially if you are a teenager because you get acne. So what is

acne? Acne is essentially an infection in the pores of your skin. So it's bacteria that

are living and feeding inside our body. And so how do we fight that? Well we'll plug it

up for one thing, but we have swelling. So we're going to increase the heat. But basically

we're going to send soldiers in there. And those soldiers are the macrophages. Those

are going to be the eaters or the big eaters. And what they're going to do is they're going

to find anything that's not part of our body and they're going to eat it. So any invader

is called antigen. And so basically what a macrophage will do is it will notice that

this is not part of our body. It will grab on to it. It will take it into the macrophage.

It'll secrete lysosomes and enzymes into it which break it up. It will lots of times present

that on its surface, but eventually gets rid of it. And so this is an actual picture of

a macrophage. And you can see it's grabbing I don't know if it's viruses or bacteria on

either side. And so that's great. But the one thing about it is it attacks anything

that's not us. In other words if you get a heart implant from someone else, or you get

a heart transplant, those macrophages are going to attack it as well. It's going to

kill that. And so we also have what's called a specific immune response. Specific immune

response is more like the spies. And so basically here's an antigen again. An antigen is going

to have specific proteins on its surface. But to fight that we use what are called antibodies.

And so the name antigen means an antibody generator. In other words it generates the

formation of antibodies. So what are antibodies? Antibodies are going to be proteins produced

by our body. And basically they all look the same. They're this Y kind of a shape. So they're

a Y shape like this. We produce almost an infinite number and an infinite variety of

them. But at the top of the Y you're going to have different shapes. In other words,

you're going to have a shape that looks like this. But you're also going to have an equal

shape that might look like this. And you're going to have an equal shape that might look

like this. And so we're going to have all of these different shapes at the top. But

we're only going to produce the shape for the one thing that we're infected by. So basically

the antibody will dock to the antigen. And when it does that basically it marks the antigens

so macrophages can find it. And it also makes it harder for them to do their job. Imagine

if I had another antibody here. And another antibody here. And another antibody here.

It's hard for them to do their job. And so when you gain immunity, specific immunity

or specific immune response what that means is you have the ability to produce these antibodies.

And that's why when you have a cold you're not going to get that same cold again. And

so basically how do we do this? Or how does this work? Well we need what are called lymphocytes.

Lymphocytes are a type of white blood cells. And so basically there are two types of lymphocytes.

There are B lymphocytes and then there are T lymphocytes which will get to in just a

second. Okay. So B lymphocytes are made in the bone marrow. And they're responsible for

a humoral response. Now this term I'm going to show you a couple more times in this podcast.

Humoral means in the fluid or in the humers of your body. So that means in the blood.

In the lymph material. In the lymph vessels. In the interstitial fluid. And so humoral

response, what I want you to think about is going to be anytime there are viruses free

in the fluids of our body. And what the B lymphocytes do is they produce antibodies.

So how does that work? Basically you have a naive B cell. It's going to sense the shape

of the antigen. I'll tell you how that works in just a second. But basically what it's

going to do is it's going to produce antibodies. And so what B lymphocytes do is they produce

all of these antibodies that are specific for the antigen. So in other words if we're

infected by this virus, we'll call this virus A1, then we're going to produce antibodies

for that specific antigen. We're also going to produce memory B cells so that we have

that immunity for the rest of our life. So those are the B lymphocytes. And so you might

think, if they attack the viruses out inside our body then what do the T lymphocytes do?

Well the T lymphocytes are responsible for cell mediated response. What does that mean?

They're going to target and kill the cells inside our body that are already infected.

Okay. So what are the T lymphocytes? Where are they made? They're made in the thymus

gland which kind of sits on the top of our heart. Basically what they do is they create

what are called kill T cells or killer T lymphocytes. And so a killer T cell is an activated T cell.

Basically what it will do is it'll find any of the cells inside our body that are infected

with a virus. It'll dock next to them and it will kill our own cells. And so it's going

to kill any of the cells inside our body that are infected by the virus. Or even cancerous

cells, it's going to kill them. So it's made in the thymus and it produces cell death inside

us. And so the T lymphocytes are responsible for this cell mediated, I mean killing cells

inside, our cells that are infected. B lymphocytes are going to be in the humers of our body.

And so if I were to summarize this a little bit, this would be the humoral response up

here. So what type of cells are responsible for that? Those is going to be the B cells.

And here's the cell mediated down here. Those are going to be the killer T cells. And so

before we get to that let's look over here on the left side. So basically what's happening?

We have an antigen that is eaten by a macrophage. That macrophage is going to chop up that antigen.

It's going to present pieces of it on its surface. And so we use a chemical called MHC2.

It's major histocompatibility complex 2. It's going to present the shape of that antigen

on it's surface and now we get this cell right here which is super important. It's call the

T helper cell. What the T helper cell is going to do is it's going to dock and it's going

to physically sense the shape of that antigen. It uses another protein called CD4. And it's

going to sense the shape of that antigen. The helper T cell is responsible for initiating

both the humoral and the cell mediated immune response. So let's see what helper T cell

is going to do. Helper T cell is going to tell that shape to the B cells. And so it

can produce more antibodies. It's going to activate macrophages. So it can kill more

of them inside the humers of our bodies. So it's responsible for that humoral response.

And the helper T cell is also going to activate the killer T cells. So they can kill the cell

mediated, or excuse me, the cell's that are infected by a virus. And so if isn't for these

guys, if it isn't for the helper T cells, we're out of luck. Now sadly, helper T cells

are the cells that are infected by HIV or people who have AIDS. And so you can see why

that's a really bad thing. Because without the helper T cells we can't fight normal infections.

So if you have HIV you don't die of that. You're dying of normal infections that we

would fight off. So let me kind of do this in cartoon style. If we were to animate it

again. So what we've got here is our antigen. Remember that's our invader. And so basically

what's going to happen is it is going to be eaten by a macrophage. So the macrophage will

take it in. It will secrete enzymes into to it which are going to digest that antigen.

It will get rid of it. But it's also going to grab on to a little bit of that. It's going

to grab on with this major MHC. It's going to take it's shape out to its surface and

now we have helper T cell. What's helper T cell going to do? Helper T cell is going to

dock with that macrophage. And it's going to sense the shape of that antigen. It's now

going to become an activated helper T cell. So thinking back to that flow chart just a

second ago, where does it go next? Well it can activate macrophages. But more importantly

it's going to activate B cells. So now we've got an activated B cell. It's going to activate

killer T cells. And now through clonal selection it's basically, they're going to make clones

of themselves. We're going to have a whole bunch of activated B cells. We're going to

have a whole bunch of activated killer T cells. So now, thinking about it, this on the top

is going to be the humoral response up here. And this down here is going to be the cell

mediated response. And so basically we can fight those antigens out here in the humers

of the body. We do that by sticking antibodies to it so macrophages can eat it, break it

down. We also inactivate them a little bit. But if you look down here, that killer T cell

is docked with a cell inside our own body that's already infected by an antigen. And

so it's going to secrete enzymes into it that are going to break that down. So it's going

to kill that. And so basically what we have done is we've killed them in the humers or

in the fluids of the body and then we've killed cells that are infected. And so this takes

awhile. But it's going to be your immune response. And it's specific to that antigen. Until we

get that specific antigen, we're not going to produce the antibodies for it. So let's

talk about a cold, because I'm getting a cold right now. So basically what happens is you're

exposed to the cold right here. The virus is going to start reproducing inside my body

and it's going to take me a little while for me to start building memory, excuse me, B

cells and killer T cells. And so there's going to be a lag time but essentially I'm going

to produce a whole bunch of antibodies and effector T cells. So that's going to increase

inside my body. And this time right here is where I actually feel like I have a cold.

So this is me feeling like I have a cold, but really what's happening is it's my body

fighting and killing off all the viruses. And so let's say I get exposed to that same

cold again in the future. Well in the future I'm going to get exposed to it. But since

I have so many antibodies I'm going to produce them so quickly. And since I have these memory

B cells and memory T cells that are just hanging out, I'm going to fight off and kill that