- [Voiceover] How does HIV actually infect our cells?
What exactly happens when you get an HIV infection?
Well, to find out, let's follow a little HIV particle,
an actual virus, through what it does
and where it goes in order to infect a human.
So let's start by drawing some skin here.
Actually, let's say this is mucus membrane.
So the soft, most areas not quite protected
like everywhere else on your body
that's covered by normal skin.
So you'd find these mucus membranes
inside your mouth or in your throat
or your noes our inside the vagina
or at the tip of the penis or in the rectum,
or anywhere in your digestive tract, really,
any and all of these are mucus membranes
and remember, this is relevant,
because the most common way HIV is spread is
by sexual contact,
which is when these areas or some of these areas
are likely to come into contact
with infected sexual fluids from each other.
And, just to jog your memory, remember
the second most common way HIV is spread
is when it's directly put into your blood
from an unsafe needle or from blood
to blood contact with an HIV infected person.
And, you know, we can sort of intuitively see
how HIV can attack our white blood cells
like our CD4 T helper cells or our macrophages
when it's directly put in the blood stream.
That bit's a little obvious.
But what's less obvious is how HIV gets
into our bodies across our mucous membranes, right?
I mean, they look like barriers to me.
Well, it turns out that we have
these really interesting little immune system cells
imbedded within and throughout
our mucous membranes.
And these are called our dendritic cells.
And these cells are really quite key
in HIV infection.
What they do is they protrude their little limbs,
essentially up to the surface of our mucus membranes
because their job is to sort of sample the environment
to look for bacteria or viruses or any other threats
to our health.
And practically speaking, what they do is
they sort of continually grab on to particles
that they find sitting on top of our membranes here
and they pull them in across the membrane
to check them out to see whether they found
a good reason to activate our immune system or not, alright?
They actually have the ability to kick off
a pretty big immune reaction depending on
what kind of particle they've found.
So what they do in a normal scenario is
that if they find something they're worried about,
maybe a virus or a bacteria or something like that,
they internalize it and then they kind of cut it
up into a few pieces and then they display
little pieces of it which, at this point,
are called antigens on their surface,
because essentially they wanna get
other immune cells attention.
They're essentially saying, "Hey, guys, look what I found!
"Let's make some immune cells that our targeted to these
"little pieces here, let's wipe this thing out."
And really, really, really importantly,
while they're doing all of this, all of this break down
and displaying of foreign invader antigens,
they're also on the move.
They've sort of picked up shop
and they're already on their way
through the blood stream, or through the lymphatic system
on their way to one of our many lymph nodes.
They usually go to the closest one.
Lymph nodes, by the way, are a really important part
of our immune system where a lot of our immune cells,
particularly our lymphocytes, hang out
and sort of filter through our body fluid
for any pathogens that they might need
to become sensitized to and respond to.
You can think of these lymph nodes
as police stations where a lot
of our cops hang out waiting to be briefed
on their next task, to be shown wanted posters,
so to speak.
This is standard practice for our dendritic cells,
by the way.
The normal sort of every day life
for these guys goes something like
they hang out in the mucousal surface,
they find pathogens, they carry them
to the lymph nodes to alert the lymphocytes.
This is just a normal day for them at this point.
The problem with HIV though is
that it's not quite like the other pathogens,
either bacteria or viruses.
I mean, some of it gets degraded, right?
Chopped up and displayed to the lymphocytes
and the lymph nodes here, sure.
But some of the HIV actually gets transmitted
to the lymph node in tact
which is no good.
And I'll tell you how that happens.
There's three sort of main ways
that our dendritic cells sort of misguidedly bring
intact HIV to our lymph nodes.
One way is by what's called an infectious synapse
where there dendritic cell essentially grabs
on to HIV particles using its plasma membrane
and then a protein on its plasma membrane
and it essentially carries it, just like this,
to the lymph node and then it sort
of hands it off, hands off the little HIV particle
to a T-cell to deal with,
which usually results in that T-cell becoming infected.
Another way is where the dendritic cell endocytosis
a group of HIV particles,
bring that endocytosed group to the lymph node
and then exocytoses them right inside the lymph node
where all you other immune cells get exposed to them
and can thus get infected as well, right?
Not very thoughtful of the dendritic cell
to do that.
And the third way is that HIV will directly infect
our dendritic cell at some point
during initial contact either at the mucus membrane
or during the period of time when the dendritic cell
is traveling to the lymph node.
And throwing an infected dendritic cell
into a lymph node is generally a terrible idea,
probably the worst idea anyone could ever have,
because after becoming infected,
the dendritic cell will start pumping out
copy after copy after copy of HIV particles
that are just as infectious as the first one.
But, you might be wondering, well,
how does infection of an immune cell by HIV
result in more copies of HIV being made?
And that's a great question
and that's actually the standard way
that viruses work.
They sort of highjack our cellular machinery
and they use it to create lots and lots
of copies of themselves that can then sort
of leave the infected cell and go on
to infect more of our other cells
to sort of repeat the whole process.
HIV though, it sets itself apart
from almost every other virus because the way it does
this highjacking is a little more different
and a lot more sneaky than other viruses.
And I'll cover how it highjacks when we look at
how HIV particles infect their preferred targets,
our T-helper cells.