So let's move on to
some of the details
surrounding the immune system.
Let's start by
differentiating between
nonspecific and
specific body defenses
and giving examples of each.
As mentioned, there's a
number of different pathogens
as indicated here,
invading enemies,
but things that can
get into our body
and produce disease, fungus,
cancer, parasites, viruses,
or bacteria.

Our body has two major
levels of immunity.
The first is
nonspecific immunity,
as indicated on the left side.
To break down that
term, nonspecific,
what it's referring
to is it's not
specific to any of these
particular invading enemies,
the fungus, cancer, et cetera.
And so the first line of defense
within nonspecific immunity
are simply barriers– like
chemical barriers– stomach
acid, and tears,
mechanical barriers–
like skin– or
reflexes– like coughing.
Each of these types
of defenses protect us
by not allowing the fungus,
parasites, bacteria, et cetera,
into our body in
the first place.
Tears wash away any
bacteria that might
get on the surface of our eye.
Mechanical barriers,
like our skin,
prevent viruses or fungus from
even getting into our body
by providing that
wall-like structure.

The second line of defense
is nonspecific immunity,
but that's involved
in protecting us
after these pathogens may
have got into our body, things
like inflammation and fever.
If some of the bacteria
actually cross our skin
and get into our body, we might
have inflammation and fever
in the area where they
actually got into the body.
We'll talk a little
bit more about that
and how it works in a bit,
but it's not a reaction
to any specific
bacteria or viruses.
But it's causing inflammation
and high temperatures
to try to kill off those
bacteria or viruses.
Phagocytes are
actually cells that
will engulf these
pathogens and eat them up.
Protective proteins
are various types
of proteins that will attempt
to destroy these pathogens
or to protect other cells from
having the pathogens attack
them.
There are also natural killer
cells, which are a type of cell
that release chemicals to
destroy invading pathogens.
Another level of immunity
is specific immunity.
And this incorporates
our third line
of defense, which are
T cells and B cells.
We'll get a little bit more
into T cells and B cells
later on in this unit.
But these are specific,
because they will attack
specific types of disease.
For example, I
might get infected
with one bacteria, where
certain T cells and B
cells will react in order
to kill off that bacteria.
I might get sick later
with another bacteria.
And the T cells from
my original infection
don't function to fight
off that type of bacteria,
but other T cells
and other B cells
will mount a response,
because they are specifically
tailored to attack that
second type of bacteria.
So continuing on, we can
again take a closer look
at nonspecific immunity.
Nonspecific immunity,
as mentioned previously,
protects the body against many
different types of pathogens.
It does not recognize
any specific pathogens
and attack them,
but has systems,
like mechanical barriers,
chemical barriers,
and reflexes,
ultimately, forming
the first line of
defense, as mentioned
in the previous slide,
that prevent pathogens
from getting into our
bodies in the first place.
Skin and mucus
membranes align things,
like our lungs and
our digestive system
are mechanical in
the sense of they
don't physically let the
bacteria or the viruses
into our underlying tissues.
Chemical barriers, like sweat
or the acidity of our stomach,
have a pH such that most
bacteria and viruses can't
live under those
acidic conditions
and therefore, chemically
break down those pathogens
before they can
get into our body.
Tears flush those
pathogens out of our eye.
On top of that,
they also secrete
a substance called lysozyme.
This discourages the
growth of pathogens
on the surface of the eye.
We also have reflexes,
like sneezing,
that when we get dust
or other pathogens
into our respiratory system,
the reaction of sneezing
helps to clear them
out of that system.
We also just took a
look at phagocytosis.
To break that word down,
phago means to eat.
Cyto refers to cell,
and osis is a process.
And so phagocytosis is the
process of cell eating.
And it's kind of indicated
in this image here.
Certain cells– and we'll
look at some in a little bit–
will undergo phagocytosis,
where, as indicated in the pink
here, we have some pathogen
that gets into our body
and the cell will wrap around
that, envelop it, and bring
that pathogen inside the
cell and then destroy it.
Inflammation and fever were
mentioned previously, as well.
And we'll take a closer
look at those in a second.
And then we have protective
proteins, like interferons.
Interferons are chemicals that
are released by cells that
have been invaded by viruses.
When a virus gets
into a cell, the cell
will release interferons
that go to other cells
and communicate that that
cell is currently infected.
It interferes with the
propagation of the virus
by telling the other cells to
ready themselves to not allow
the virus to enter into them.
Complement is made up
of a number of chemicals
that are within our body, that
when we have a bacteria go
into our body, the complement
will attach to the bacteria
and punch holes into the surface
membrane of the bacteria,
thus breaking down the
bacteria, because it
can no longer function properly
with these holes in the plasma
membrane.
We also mention
natural killer cells,
which are a special
type of cell that
acts non-specifically to
kill a variety of cells.
It's not specific to
any type of pathogen
that's getting into our body.
And although, we won't look
at it much in this course,
they are nonspecific immunity.
And specifically,
all of these together
form the second line of defense.

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