One of the things that is not exactly unique,
but is rather special about the immune
response is that the cells of the immune
system actually move around the body.
They?re not just sitting in one location,
but they can travel around the body.
And essentially they do this
using two different circulation
systems - the blood circulation
and the lymphatic circulation.
And these are connected to each other,
so the cells can move
around both the blood circulation
and the lymphatic circulation.
And this allows cells of the immune system to
actually get to the location where the infection is.
It?s not good having an infection in your big toe if the
cells that you need to fight that infection hasn?t--
under your armpit for example, the cells of the immune
response need to get to the location where the infection is.
And that?s why they
circulate around the body.
We already heard earlier how there are
many different types of pathogens that
the immune response needs to detect
and mount an immune response against.
Viruses, bacteria, fungi, parasites,
and I mentioned that these
can be very small like a virus or very big like a parasite.
They can be living inside
cells, in the case of viruses.
They spend some of their life cycle inside
a cell, but some of it outside of a cell.
So I?m sure you will not be surprised to hear
that there?s not a single strategy that the
immune system uses to eliminate pathogens,
but actually several different strategies.
The detailed for the particular type of
pathogen that needs to be eliminated.
Phagocytosis - this is a process whereby pathogens are
engulfed by cells that we refer to as phagocytic cells.
There are two major sorts of
phagocytic cell in the immune response.
One is called a neutrophil and the one
that you can see here, a macrophage.
So the phagocytic cell engulfs,
takes up the pathogen and then
subsequently having taken up the pathogen, it destroys it.
So it takes it up into what is
called a phagocytic vacuole.
And then the phagocytic cell releases toxic
molecules onto the pathogen which destroys it.
Obviously some pathogens are
larger than our own cells.
I?ve mentioned earlier, a
big tapeworm in your gut.
Some other types of parasites are
much bigger than the phagocytic cells.
And clearly, the phagocyte can only take
up organisms that are smaller than itself.
So, other strategies are needed for larger
organisms or other types of pathogen.
One of those strategies is for
cells of the immune response to
release molecules that are toxic to
the pathogen, can kill the pathogen.
Another strategy is required when pathogens
actually hide within cells of the body.
And all viruses have to spend some of their
life cycle inside our own body cells.
They?re not able to replicate their nucleic acid without
living at least for part of their life inside our own cells.
So a number of different strategies are
employed: phagocytosis, the release of molecules
And they are then hidden from many
aspects of the immune response.
that can kill a pathogen and cells that can kill
our own body cells early on in an infection.
So how does the immune system deal
with that, you may be wondering?
Well, there are a number of different cell types that can
actually kill our own cells when those cells become infected.
Seems a bit harsh, it seems a bit
brutal to start killing our own cells.
But as long as the immune system does that very early on
in an infection, it?s usually fine, it?s usually okay.
We can regenerate a
few of our own cells.
It?s not a big problem for us.
And it?s a way that the immune system
has evolved to deal with pathogens
that are trying to avoid the immune
response by hiding inside our cells.
It?s a very effective way of
dealing with that situation.
And the cell of the immune system
will then kill the infected cell and
other parts of the immune system can
then deal with the killed cell.
Remove it, remove any pathogen that gets released
from those cells following the cell death.
So,a number of different strategies are imployed:
Phagocytosis, the release of molecules that can kill pathogen
and cells that can kill our own body cells
early on an infection.