- [Narrator] Another nonspecific defense are called
natural killer cells.
These natural killer cells,
I'm willing to bet you probably figured out what they do.
Sure, they kill.
But the idea is, they're a type of lymphocyte.
Remember, a lymphocyte is a white blood cell,
but they attack tumor cells and virus infected cells.
They have a pretty specific function.
One reason why they're important is a virus can only
make more viruses when it's inside a cell.
If the natural killer cells go after those infected
cells, they might prevent them from making more viruses.
Then we have these complement proteins.
I already kind of mentioned those a few slides ago.
Complement proteins can tag bacteria, but also lyse them,
which means break them open.
So lyse invading bacteria, or break those bacteria open.
First off, complement proteins will get near a bacteria.
They'll find it.
They will create holes in the bacteria as they find them.
The problem with creating holes in a bacterium,
is that water and salt will rush into the bacterium.
As they rush into the bacterium,
the bacterium is going to swell and swell,
and eventually it will rupture.
It just cannot take all the water and salt going into it
from the surrounding environment.
We also have some interferons.
If you think you know what that one means by interferons,
it interferes, you're exactly right.
Interferons are antiviral proteins.
They're meant to try and prevent viral spreading.
Fever. I think everyone has had a fever.
It's not the most pleasant thing in the world,
but it's meant to help your body.
As your body warms up with a fever,
it's your body's way of helping to fight the infection
that you have.
Helping to give your body cells a little extra edge
of responding a little bit quicker.
So fevers, even though they're not great,
they do their job.
With all those nonspecific,
with all of those first line defenses,
you'd think we'd be pretty protected.
But amazingly enough, we still get infections.
So we have this third line defense.
The third line is kind of what you think of
as the immune response.
It's going to recognize and it will target
the pathogen that's after the body.
That means this is a specific defense.
Since you have to be able to recognize it
and then target it you must know exactly
what the pathogen or foreign substance is.
Therefore, this immune response,
this specific defense, has a memory.
It can remember any exposure you've had in the past.
Think about chicken pox again.
If you've ever had chicken pox,
or you had the vaccine for chicken pox,
odds are you probably didn't actually get full-blown
chicken pox a second time,
or even a first time after the vaccine.
Occasionally, there might be that person
that has a second bout with chicken pox.
But this is the reason why you typically
don't get a second turn.
The first time you see that virus,
your body will put it into memory.
It will remember that virus.
So if it sees the virus again, it's going to very quickly
know how to get rid of it.
The only problem is this memory only works
on the exact same infection.
It has to be the exact same bacteria
or the exact same virus.
Any tiny change doesn't work.
An example is the flu.
Sometimes if the person is very lucky they might
get two or three strains of the flu in one flu season.
How can that happen?
They're each different strains.
So even though your body might have fought off one strain,
you might get a second strain,
because it doesn't recognize it.
So this memory is a great thing but it has limitations.
These limitations, thankfully, do not have a problem
with our self cells.
Self cells, or our own personal cells,
have their own self surface marker.
That means this immune response,
as long as it's healthy and functions properly,
can recognize your own cells from foreign invaders.
It can also distinguish between your healthy cells
and any abnormal cells like a tumor.
If you're saying, hold on, if it can distinguish
between healthy and tumor cells,
why can't your body get rid of tumors?
Simple fact is, tumors grow extremely quickly
and sometimes it's very hard for your body
to even find them.
It might be coated with cells
that are your own healthy cells.
Unfortunately there are ways that these pathogens,
these problems, can basically trick your immune system.
What we have to look into is targeting.
How does your body know what to target?
We need to look at what we call antigens.
Antigens are found on the cell surface.
It's a way to identify what the cell is.
You have antigens that are specific to your own body.
Your cells recognize your antigens.
They don't attack them.
But if you have an antigen in your body
that your body does not recognize,
it means it's a foreign cell.
It might be a bacteria, virus, or who knows what.
We're looking at these MHCs,
major histocompatibility complexes.
Basically, that's the fancy name for your own antigens.
This is what allows your self to tell that it's your self,
and not something else.
So the MHC proteins are what tell you these cells
are your own cells.
It's kind of important for your body to be able
to recognize these MHCs.
How do you recognize they're foreign?
You need to have cells that can go and look for them.
B lymphocytes. These are dealing with your
antibody-mediated immunity.
What that means is, the B lymphocytes are going to
eventually produce antibodies.
Antibodies will recognize antigens.
It kind specific, like a lock and key.
Only one particular key can open a certain type of lock.
We have only a certain type of antibody can find
a certain type of antigen.
These are proteins that are made by B lymphocytes,
not directly, because there's a step in between,
but for all intents and purposes the B lymphocytes
will eventually produce them.
They recognize certain antigens,
whether they are foreign antigens or in certain
problematic cases your own antigens.
These B lymphocytes also can be active against viruses,
bacteria, foreign molecules, any of these.
Whenever a B cell is activated, it means
they found a foreign antigen.
It means they found an antigen that they don't recognize.
Therefore, they activate to make antibodies against
that new foreign antigen.
Perhaps your body is in catch-up mode now.
Even though this is a third line defense,
that means that once these particles reach
this deep into your body,
your B cells can look for them and recognize them
and say hey, that's a bacteria.
We need to get rid of it.
It will then activate the B cells, get it working,
and try to find antibodies to make the antibodies
for the antigen.
T lymphocytes are another version of lymphocyte.
While the B lymphocytes are dealing with more
of the antibodies, which can recognize foreign antigens,
T lymphocytes are dealing with cell-mediated immunity.
They go right out there, attacking.
They directly attack the foreign cells.
But they also will help activate your immune system
and kind of coordinate the response.
T lymphocytes are pretty important.
You might have heard of the T lymphocytes before
in relation to HIV/AIDS.
HIV/AIDS, a lot of times, will have T cell counts.
T cells are T lymphocytes
and when those T cells drop down to a certain level,
a lot of times the body becomes compromised.
Once these T lymphocytes are low enough,
the body is compromised because it just cannot
fight off an infection.
There's not enough immune system left to fight,
which means that even the most basic common cold,
could come in and cause real problems,
because the body can't fight it off.
These T lymphocytes are going to fight against
parasites, viruses, fungi, intracellular bacteria,
cancer cells, cells with anything other than self.
Pretty much, that list can be summed up as in,
anything that does not have your own self marker.
So, it does not recognize the MHC.
T lymphocytes are going to take it on.
Here's a figure out of your book,
but don't worry about all the steps.
Just get the basic idea.
You start off with the bacterium.
The B cell identifies, saying huh, okay.
It forms memory cells and forms antibodies.
For that foreign particle,
doesn't matter if it's bacteria, virus,
whatever it is, the key thing here is,
for each recognized foreign antigen,
you're making antibodies against it,
and you'll make memory cells against it.
That way, if you ever see it again,
you'll know how to fight it.
Here's the way it works.
Our pathogen here, is that kind of pink pill shape.
You can see the antibodies are the green Y's.
The green Y's, these antibodies, were made to identify
this particular pathogen.
So what happens is, the antibodies bind to the pathogen,
identifying it as that particular pathogen,
as a foreign particle.
What the antibodies do then is,
either some cell that is phagocytic will come along,
recognize the antibody and destroy the bacteria.
Or the antibodies start to clump together.
By clumping numerous pathogens together,
the pathogen really can't function.
So either way, by clumping together or by identifying
it to be destroyed to a phagocytic cell,
you're getting the bacteria or the pathogens out of the way.
How about some T cells?
There are different types of T cells.
T cells will attack, they'll coordinate.
But we also have helper T cells.
The helper T cells, really the ones that are
going to kind of direct the immune response.
They're going to stimulate various parts
of the immune system.
They are going to go out there and literally help
activate everything, get everything ready,
get everything going.
Cytotoxic T cells.
Just like the name implies, cytotoxic,
cyto is cell, toxic is deadly.
So cell death. T cells.
Cytotoxic T cells will attack and destroy tumor,
viral-infected cells or anything foreign for that matter.
Memory T cells.
These are the ones that are made to help store
the information of what was fought off.
So once you fight off, once you get rid of an infection,
the memory T cells are left behind.
If you ever see that exact same infection again,
the memory T cells are reactivated and you can
fight off that infection much quicker.
Here you can see your cytotoxic T cells.
The one on the left is an artistic rendition.
The one on the right is an actual micrograph.
The bacteria cells aren't really blue and pink,
that's just adding color.
But you can see how those cells are latching on,
recognizing and attacking on the surface of the pathogen.