Hi. It's Mr. Andersen and welcome to Biology Essentials video number 23. This
is on plant and animal defense mechanisms. Plants and animals are constantly trying to
be invaded. And they a couple of different strategies they can use to defend themselves.
Plants will use nonspecific defense in general. They don't have an immune response per se.
What they'll do is actually kill themselves and I'll explain that in just a second. And
then we have specific response. In other words we have a, mammals are going to have a complex
immune response so they can identify organisms that have infected them in the past. So this
is chicken pox here. And if you received chicken pox when you were young you will never get
it again because you have this immunity that'll last a lifetime. And we can actually give
you a vaccine. A lot of kids today are vaccinated for chicken pox and so they'll never get it.
And so today I'm going to talk about first of all pathogens. Pathogens are things that
invade us. In general pathogens are bad. So we don't want them to invade us. And so to
defend ourselves we have a couple of different defense mechanisms. Some are nonspecific.
Nonspecific defense is going to kill anything that invades us. And so for us if you get,
let's say you prick your finger with a needle, it's going to swell up and what you're going
to do is you're going to kill anything that moves inside your body. It doesn't matter
what it is, you're just going to destroy it. And the skin actually forms a protective
barrier around us and gives us quite a bit of that nonspecific defense. But we're probably
not familiar with plants and how they defend themselves. They use something called a hypersensitive
response. They can sense a pathogen invading them. They also have cell walls. Obviously
things that protect them, but once they're invaded, they have a pretty dramatic defense
mechanism. In us we use a specific defense. And so we're going to identify actual pathogens
that have invaded us and we're going to target them specifically. We break our specific defense
into two different types. We have humoral response. Humoral response is going to be
found in the humors, or the liquids of your body, for example in your blood or in your
lymphatic material. It's generally governed by B lymphocytes. And what they do is they
produce antibodies to mark antigens for destruction. We can hold on to memory B cells for the rest
of our life. And that's why you can never get chicken pox again. We also have a cell
mediated response, I'm missing an "e" here. Cell mediated response is going to be targeting
cells inside our body that are actually infected. So for example, chicken pox is going to infect
cells with viruses and so we want to target them and kill them as well. The major cell
is going to be a type of white blood cell called the cytotoxic T cell. What it does
is it kills all the infected cells in your body. So let's get to some of the specifics.
Let's first start with the nonspecific response. Nonspecific remember is going to kill any
pathogen that invades the organism. So for us our nonspecific response is going to be
things like our skin, saliva. We're going to have normal bacteria that live on our body
just to kind of crowd those off. But right here what we're looking at is a plant defense.
And plants show what's called hypersensitive, or really really sensitive, response. And
so think of this as a leaf of like a tobacco plant for example. And this is a bacteria
that's trying to infect that plant. The bacteria is going to give off proteins. And we'll talk
more about those later. But those proteins are going to be sensed by the plant. And so
what I tried to just kind of simplify is this is the chromosome, a simple chromosome inside
a plant. And what they have is a gene called a R gene. And the goal of the R gene is to
sense those invading proteins. And so if there's ever a match between those proteins and the
R gene, then plants are going to undergo what's called a hypersensitive response. And so first
you have to have a sensation. Next thing that they're going to do is they're actually going
to set off what's called an oxidative burst. It's pretty cool name. But what it really
means is they're going to free up all of the excess highly reactive oxygen they have inside
the cell. And so what that eventually does is apoptosis. It kills the plant cell. So
they're going to destroy the cells that sense, even though it might not be infected, even
though this pathogen may not of even infected those cells, just picking up those proteins
is going to kill the cell. And so we're going to destroy those cells and it's also going
to give off a number of proteins that are going to affect the cells right next to it.
So not only have we killed these cells, but it's going to change, proteins are going to
be released that are going to change the cell wall of all the adjacent cells. And so what
would that look like? Well if this is a tobacco plant, just having pathogens on its surface
is going to cause that, all those cells inside this infected area, to actually die. They
kill themselves. And you'll see this on leaves if you start looking for it. They'll kill
themselves. Those eventually die. And then all of the cells around it, so all the cells
out here are going to change their appearance and so they're not able to be infected. And
so that's a hypersensitive response. It's going to target any kind of invader. So whatever
bacteria protein, fungus, virus, whatever is trying to get in, it's going to destroy
the cells and then it's going to form lesions around it so it can't be infected. Now, it
doesn't have memory. And so the next time it's infected by that pathogen it has to undergo
hypersensitive response as well. In mammals we have that. In other words swelling, inflammation.
All those things are going to be a nonspecific response. But what's cool about us is that
we have a specific response. In other words once we're, once we get a cold, we'll never
get that same cold again. Once we get chicken pox, we'll never get chicken pox again. Now
it could manifest itself as some other kind of a disease, like shingles or the cold could
mutate, but in general, once we have that infection, we can't get it again. So this
guy right here is a great example of immune response. This is Edward Jenner. And back
in the day the number of people that we being killed by small pox was unbelievable. We're
talking about millions of people that were being infected by small pox. I didn't even
put pictures of people with small pox on here because it's pretty revolting actually. Awful
way to die, to die of small pox. But this is what the virus looks like. And what Jenner
noticed is that women that were milkmaids, that were picking up a disease called cow
pox, cow pox will form on the utters of a cow, but it was also transferred to these
women. They were given an immunity to small pox. In other words, somehow by picking up
the cow pox disease, you couldn't get small pox as well. Now Jenner didn't understand
antibodies and how they work. We now understand what happens is that these Y shaped proteins
were being secreted in the body of a person who gets small pox. And what they do is they
mark the virus. Now not only do they mark the virus, so a big thing like a macrophage
can actually eat it, but they'll actually adhere this one to this one. So they stick
the viruses together and then they mark them. And so that they'll be eaten by a macrophage
and destroyed. And so proteins are going to look like this. So this would be an antibody.
These are secreted by B cells inside our body. But here's the active part. In other words
there's going to be a specific shape on the antibody. And if that specific shape meets
a specific shape on the antigen, now I should talk about what an antigen is. An antigen
is going to be a antibody generating organism. And so it could be things like a virus. It
could be things like a bacteria. It could be things like a fungus. All of these things
have proteins on their surface and if the protein fits a specific antibody inside our
body then we're going to make a defense against them. And so these antibodies will stay with
us for the rest of our life and so we can't get that. So if you think about it, how did
Edward Jenner, who eventually added cow pox to people, inoculated people with cow pox
so they couldn't get small pox, how did that work? He wasn't giving them small pox. He
was just giving them cow pox. Well, the cow pox virus had proteins on its surface that
looked enough like the small pox virus so that it actually made antibodies. And so it
protected against that. And so the specific response again is targeting an invader so
that we never get that same disease again. We can never get that same infection again.
And so this is the best kind of a diagram. It's really complex, the immune system. And
I'm not going to get too in depth on it, but essentially what you have is an antigen. Remember
that's going to be an infector or an invader. It's going to be eaten by a white blood cell
or a macrophage inside our body. It's going to recognize that it's not us. And it's going
to present proteins of that antigen on its surface. And so that actual cell is called
an antigen presenting cell. And so it's going to chop up that antigen. It's going to present
its proteins on its surface. And then we come to the most important cell in all of the immune
response. It's called helper T cell. What the helper T cell is going to do is it's going
to recognize the shape of that antigen. It's going to recognize the shape. It's going to
activate B cells to make antibodies. And those antibodies remember are going to adhere to
the antigen. It's going to make two types of B cells. It's going to make plasma B cells.
A plasma B cell is going to be a cell that actually makes antibodies. And the other type
of B cell it's going to make is something called a memory B cell. Memory B cells are
going to be B cells that have antibodies on their surface. And we keep those for the rest
of our lives so we can produce more antibodies really, really quickly. So it's going to sense
the shape, the helper T cell and make these B cells. So it can make antibodies. It's also
going to be, helper T cell, is going to activate killer T cells. And so those killer T cells
are going to affect infected cells inside our body. And so remember there are two types
of response. There's a cell mediated response. Whose in charge of that? Those are going to
be the T cells. The cytotoxic or the killer T cells. So all the infected cells inside
our body, now that they sense the shape of that antigen, are going to kill our own cells
that are infected. And also we have the humoral response. In other words we're going to use
B cells to make antibodies, to make these memory B cells, and so that we can target
the actual antigens. And so humoral response is going to target antigens when they are
in the humors of our body. And cell mediated response is going to target cells inside our
body that have the antigens actually inside of them. And it's going to kill those cells.
Now if you were to talk about what is a nasty disease that we're fighting across the world
right now? What's the big pandemic that we have today? That would be HIV. Or human
immunodeficiency virus. Now what makes human immunodeficiency virus really really nasty?
Well it's a retro virus. It does high mutation rates. But the biggest one is that HIV is
actually going to infect helper T cells. And so by killing helper T cells, it stops the
humoral response. It stops that cell mediated response as well. So let's put that on a timeline.
Let's say you get a cold today. So if you get exposed to a cold today, days later the
virus count is going to increase, but your body is going to sense that and it's going
to make antibodies. It's going to make B cells. It's going to make T cells. It's going to
increase the amount of those. And so we can fight the infection. And so really you don't
sense getting a cold. What you sense normally is getting better from a cold. And so this
area right here is going to be killing of all those viruses. Then we decrease the amount
of antibodies. Let's say you get exposed to that same cold weeks later. Well you have
way more antibodies already. And those antibodies are more better, in other words they function
more correctly and quickly. And so you will fight that infection off. But you don't even
now that you did. And even years later you could have a reinfection years later and again
we have so much of those memory cells after two infections that you're not even going
to feel getting that cold. Unless there's a change in that antigen, or a change in that
pathogen to the point where their shape is actually changing, we have antibodies for
it. And so again that's a specific response. We're targeting these specific invaders inside
our body. But again defense mechanisms are set up to prevent infection due to pathogens.
And I hope that's helpful.