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- [Narrator] Another
nonspecific defense are called

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natural killer cells.

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These natural killer cells,

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I'm willing to bet you probably
figured out what they do.

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Sure, they kill.

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But the idea is, they're
a type of lymphocyte.

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Remember, a lymphocyte
is a white blood cell,

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but they attack tumor cells
and virus infected cells.

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They have a pretty specific function.

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One reason why they're
important is a virus can only

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make more viruses when it's inside a cell.

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If the natural killer cells
go after those infected

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cells, they might prevent
them from making more viruses.

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Then we have these complement proteins.

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I already kind of mentioned
those a few slides ago.

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Complement proteins can tag
bacteria, but also lyse them,

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which means break them open.

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So lyse invading bacteria,
or break those bacteria open.

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First off, complement proteins
will get near a bacteria.

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They'll find it.

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They will create holes in the
bacteria as they find them.

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The problem with creating
holes in a bacterium,

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is that water and salt will
rush into the bacterium.

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As they rush into the bacterium,

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the bacterium is going to swell and swell,

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and eventually it will rupture.

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It just cannot take all the
water and salt going into it

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from the surrounding environment.

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We also have some interferons.

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If you think you know what
that one means by interferons,

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it interferes, you're exactly right.

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Interferons are antiviral proteins.

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They're meant to try and
prevent viral spreading.

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Fever. I think everyone has had a fever.

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It's not the most pleasant
thing in the world,

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but it's meant to help your body.

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As your body warms up with a fever,

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it's your body's way of
helping to fight the infection

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that you have.

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Helping to give your body
cells a little extra edge

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of responding a little bit quicker.

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So fevers, even though they're not great,

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they do their job.

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With all those nonspecific,

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with all of those first line defenses,

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you'd think we'd be pretty protected.

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But amazingly enough,
we still get infections.

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So we have this third line defense.

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The third line is kind
of what you think of

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as the immune response.

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It's going to recognize and it will target

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the pathogen that's after the body.

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That means this is a specific defense.

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Since you have to be able to recognize it

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and then target it you must know exactly

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what the pathogen or foreign substance is.

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Therefore, this immune response,

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this specific defense, has a memory.

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It can remember any exposure
you've had in the past.

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Think about chicken pox again.

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If you've ever had chicken pox,

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or you had the vaccine for chicken pox,

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odds are you probably didn't
actually get full-blown

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chicken pox a second time,

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or even a first time after the vaccine.

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Occasionally, there might be that person

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that has a second bout with chicken pox.

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But this is the reason why you typically

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don't get a second turn.

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The first time you see that virus,

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your body will put it into memory.

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It will remember that virus.

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So if it sees the virus again,
it's going to very quickly

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know how to get rid of it.

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The only problem is this memory only works

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on the exact same infection.

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It has to be the exact same bacteria

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or the exact same virus.

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Any tiny change doesn't work.

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An example is the flu.

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Sometimes if the person
is very lucky they might

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get two or three strains of
the flu in one flu season.

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How can that happen?

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They're each different strains.

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So even though your body might
have fought off one strain,

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you might get a second strain,

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because it doesn't recognize it.

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So this memory is a great
thing but it has limitations.

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These limitations, thankfully,
do not have a problem

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with our self cells.

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Self cells, or our own personal cells,

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have their own self surface marker.

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That means this immune response,

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as long as it's healthy
and functions properly,

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can recognize your own
cells from foreign invaders.

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It can also distinguish
between your healthy cells

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and any abnormal cells like a tumor.

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If you're saying, hold
on, if it can distinguish

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between healthy and tumor cells,

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why can't your body get rid of tumors?

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Simple fact is, tumors
grow extremely quickly

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and sometimes it's very hard for your body

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to even find them.

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It might be coated with cells

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that are your own healthy cells.

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Unfortunately there are
ways that these pathogens,

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these problems, can basically
trick your immune system.

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What we have to look into is targeting.

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How does your body know what to target?

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We need to look at what we call antigens.

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Antigens are found on the cell surface.

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It's a way to identify what the cell is.

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You have antigens that are
specific to your own body.

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Your cells recognize your antigens.

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They don't attack them.

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But if you have an antigen in your body

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that your body does not recognize,

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it means it's a foreign cell.

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It might be a bacteria,
virus, or who knows what.

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We're looking at these MHCs,

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major histocompatibility complexes.

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Basically, that's the fancy
name for your own antigens.

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This is what allows your self
to tell that it's your self,

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and not something else.

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So the MHC proteins are
what tell you these cells

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are your own cells.

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It's kind of important
for your body to be able

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to recognize these MHCs.

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How do you recognize they're foreign?

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You need to have cells that
can go and look for them.

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B lymphocytes. These are dealing with your

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antibody-mediated immunity.

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What that means is, the B
lymphocytes are going to

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eventually produce antibodies.

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Antibodies will recognize antigens.

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It kind specific, like a lock and key.

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Only one particular key can
open a certain type of lock.

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We have only a certain
type of antibody can find

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a certain type of antigen.

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These are proteins that
are made by B lymphocytes,

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not directly, because
there's a step in between,

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but for all intents and
purposes the B lymphocytes

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will eventually produce them.

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They recognize certain antigens,

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whether they are foreign
antigens or in certain

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problematic cases your own antigens.

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These B lymphocytes also can
be active against viruses,

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bacteria, foreign molecules, any of these.

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Whenever a B cell is activated, it means

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they found a foreign antigen.

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It means they found an antigen
that they don't recognize.

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Therefore, they activate
to make antibodies against

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that new foreign antigen.

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Perhaps your body is in catch-up mode now.

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Even though this is a third line defense,

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that means that once these particles reach

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this deep into your body,

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your B cells can look for
them and recognize them

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and say hey, that's a bacteria.

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We need to get rid of it.

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It will then activate the
B cells, get it working,

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and try to find antibodies
to make the antibodies

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for the antigen.

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T lymphocytes are another
version of lymphocyte.

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While the B lymphocytes
are dealing with more

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of the antibodies, which can
recognize foreign antigens,

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T lymphocytes are dealing
with cell-mediated immunity.

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They go right out there, attacking.

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They directly attack the foreign cells.

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But they also will help
activate your immune system

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and kind of coordinate the response.

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T lymphocytes are pretty important.

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You might have heard of
the T lymphocytes before

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in relation to HIV/AIDS.

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HIV/AIDS, a lot of times,
will have T cell counts.

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T cells are T lymphocytes

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and when those T cells drop
down to a certain level,

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a lot of times the body
becomes compromised.

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Once these T lymphocytes are low enough,

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the body is compromised
because it just cannot

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fight off an infection.

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There's not enough immune
system left to fight,

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which means that even the
most basic common cold,

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could come in and cause real problems,

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because the body can't fight it off.

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These T lymphocytes are
going to fight against

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parasites, viruses, fungi,
intracellular bacteria,

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cancer cells, cells with
anything other than self.

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Pretty much, that list
can be summed up as in,

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anything that does not
have your own self marker.

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So, it does not recognize the MHC.

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T lymphocytes are going to take it on.

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Here's a figure out of your book,

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but don't worry about all the steps.

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Just get the basic idea.

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You start off with the bacterium.

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The B cell identifies, saying huh, okay.

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It forms memory cells
and forms antibodies.

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For that foreign particle,

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doesn't matter if it's bacteria, virus,

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whatever it is, the key thing here is,

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for each recognized foreign antigen,

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you're making antibodies against it,

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and you'll make memory cells against it.

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That way, if you ever see it again,

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you'll know how to fight it.

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Here's the way it works.

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Our pathogen here, is that
kind of pink pill shape.

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You can see the antibodies
are the green Y's.

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The green Y's, these antibodies,
were made to identify

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this particular pathogen.

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So what happens is, the
antibodies bind to the pathogen,

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identifying it as that
particular pathogen,

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as a foreign particle.

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What the antibodies do then is,

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either some cell that is
phagocytic will come along,

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recognize the antibody
and destroy the bacteria.

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Or the antibodies start to clump together.

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By clumping numerous pathogens together,

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the pathogen really can't function.

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So either way, by clumping
together or by identifying

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it to be destroyed to a phagocytic cell,

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you're getting the bacteria or
the pathogens out of the way.

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How about some T cells?

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There are different types of T cells.

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T cells will attack, they'll coordinate.

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But we also have helper T cells.

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The helper T cells,
really the ones that are

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going to kind of direct
the immune response.

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They're going to stimulate various parts

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of the immune system.

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They are going to go out
there and literally help

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activate everything, get everything ready,

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get everything going.

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Cytotoxic T cells.

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Just like the name implies, cytotoxic,

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cyto is cell, toxic is deadly.

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So cell death. T cells.

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Cytotoxic T cells will
attack and destroy tumor,

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viral-infected cells or anything
foreign for that matter.

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Memory T cells.

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These are the ones that
are made to help store

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the information of what was fought off.

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So once you fight off, once
you get rid of an infection,

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the memory T cells are left behind.

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If you ever see that exact
same infection again,

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the memory T cells are
reactivated and you can

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fight off that infection much quicker.

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Here you can see your cytotoxic T cells.

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The one on the left is
an artistic rendition.

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The one on the right is
an actual micrograph.

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The bacteria cells aren't
really blue and pink,

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that's just adding color.

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But you can see how those
cells are latching on,

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recognizing and attacking on
the surface of the pathogen.