So, because there are all these different strategies,
the immune response needs to coordinate its strategy.
And of course the immune
system is not like people.
It doesn’t have a brain, it doesn’t think
in the way that you and I can think.
But you can use this analogy: a group of
individuals getting together and discussing
what is going to be the best way of dealing
with a particular type of pathogen.
And essentially, this is what
cells of the immune system do.
They get together particularly in the secondary
lymphoid tissues and they work out what is
going to be the best strategy to deal with a
particular pathogenic threat that is present.
Once that strategy is agreed upon, there
are orders given and cells will be sent
out from the secondary lymphoid tissues to
go to the place where the infection is.
And molecules will be released that can
also distribute themselves via the lymphatic
system and via the blood circulation to
go to the location where the pathogen is.
So the troops are dispatched from
the secondary lymphoid tissues,
and they will hopefully be successful in defeating the enemy.
So how does this coordinated
response actually work in practice?
You may be wondering, well that’s all
very well, but how does it really work?
Well, there needs to be communication very
clearly and we can break down communication
in the immune response into two fundamentally
different ways of interacting.
Different cells of the immune
response interacting with each other.
One is where molecules on the surface of one cell recognize
molecules on the surface of another cell, and following this
mutual recognition, signals are sent into either one of the
cells or very often into both cells causing cell activation.
So this is a cell-cell contact
If you have a molecule on the surface
of one cell binding to the molecule on the
surface of another cell, clearly those
two cells have to be in cell contact.
That’s not the only way in which cells of the
immune response interact with each other.
Second way is that they can interact
via the secretion of molecules.
So, one particular cell of the immune
response will release molecules that
are detected by receptors for those
molecules on the surface of another cell.
And we can see that very clearly
on this particular slide.
So, via these cell-cell contact dependant
mechanisms and via the release of soluble molecules,
a coordinated response can be generated which
hopefully will eliminate the pathogen.
But we don’t want the response
to carry on forever and ever.
I mean once the pathogen has been cleared, if you have a
cold for example, and you’re infected with the cold virus,
once the immune response has done its job you don’t want
the response against the cold virus to carry on and on.
And the same of any
other type of infection.
So, there needs to be some way of
dampening down the immune response
or ceasing the immune response once
the pathogen has been eliminated.
Now to a great extent, that actually happens
because antigen drives the immune response.
In other words, the pathogen is actually
what is stimulating the immune response.
If the response is successful,
you eliminate the pathogen.
So there’s no longer this
drive on the immune response.
It’s a bit like the accelerator in a car,
you have your foot on the accelerator.
If you take your foot off the accelerator,
then the car will slow down.
It’s a little bit like
that with an infection.
Once the infection is gone, then it’s like
the foot no longer being on the accelerator.
So, if the response is successful,
the stimulus is eliminated and
therefore there is no longer that
antigenic stimulation of the response.
However, there’s also additional mechanisms
that suppress the residual immune response.
So we have what are called regulatory cells that make sure that
once the response is no longer required, it’s finished with.
And then the immune response can go on and deal
with other pathogens that may come along next week.
Maybe you had a cold this week,
that’s fine, you got rid of that.
But maybe next month you’ll get a
sore throat and you need the immune
response to focus its efforts on getting
rid of the sore throat infection.
So I hope you have enjoyed listening to this session and that you
have learnt some of the basics of the immune response, and can
go on and now learn in more detail about the different cells
and molecules that have evolved to protect us from infection.