what's true in World of Warcraft is also
true in your immune system to defeat
your enemy you have to know your enemy
uncover its weaknesses learn how to see
it before it sees you we've already
talked about how your innate defense
system keeps outer quietly neutralizes
pathogens without too much fuss but
sooner or later a threats gonna come
along that's stronger than what the
first responders can handle that's when
it's time for the adaptive or acquired
immune system to step in while your
innate system takes its zero-tolerance
policy very seriously and tries to toast
any foreign microbe that it encounters
your adaptive system does things
differently it has to be expressly
introduced to a specific pathogen and
recognize it as a threat before it will
attack as its name suggests you're not
born with a working adaptive immune
system it's slow to act in part because
it takes time for it to shake hands with
so many pathogens and get to know them
these introductions may be organic like
touching a dirty faucet in the bathroom
or walking into a sneeze cloud or they
may be premeditated which is why
vaccination is pretty much the greatest
thing to happen to medicine ever but
once it's been introduced to a potential
threat your adaptive defenses never
forget it and this ability to remember
specific pathogens is one of the key
differences between the adaptive and
innate defenses another main difference
is that adaptive immunity is systemic
rather than being restricted to a
particular infection in say a sinus or a
sliced finger your adaptive system can
fight throughout your whole body at once
and does this by deploying one or both
of its separate but cooperating defenses
your humoral immunity and your cellular
defenses your humoral immunity which you
might not have heard of before works by
dispatching important proteins that I'm
sure you have heard of antibodies
they're made by special white blood
cells and they patrol the body's humors
or fluids like blood and lymph where
they combat viruses and bacteria moving
around the interstitial space between
your cells much of what you know or have
heard about or think of when your immune
system comes up actually has to do with
your humoral immunity it's why if you
had mumps as a kid you probably don't
have to worry about getting it again for
the rest of your life it's also why
doctors and nurses and patients who have
been infected with the Ebola virus a
disease once thought to be incurable
have lived to tell about it and it's why
vaccinations work
whether you're protecting yourself from
infections or playing an MMO one of the
first steps in any good defensive
strategy is to be able to tell your
friend from your foe and in the case of
your immune system that means being able
to identify antigens an antigen could be
an invader from the outside world like a
bacterium virus or fungus or it could be
a toxin or a diseased cell within your
own body but in any case antigens are
large signaling molecules not normally
found in the body and they act as Flags
that get the adaptive immune system
railed up so let's say flu virus gets
inside of you and it's floating around
trying to find a good host cell to start
multiplying inside of before it finds
that cell hopefully it will be paid a
visit by one of the stars of your
humoral response a B lymphocyte like all
blood cells these guys originating your
bone marrow but unlike other white blood
cells they also mature in the bone
marrow too and as a b-cell matures it
develops the ability to determine friend
from foe developing both
immunocompetence or how to recognize and
bind to a particular antigen as well as
self tolerance or knowing how to not
attack your body's own cells once it's
fully mature a B lymphocyte displays at
least 10,000 special protein receptors
on its surface these are its membrane
bound antibodies all B lymphocytes have
them but the cool thing is every
individual loop the site has its own
unique antibodies each of which is ready
to identify and bind to a particular
kind of antigen that means that with all
of your B lymphocytes together it's like
having two billion keys on your immune
systems keychain each of which can only
open one door so part of your immune
systems strategy is just to win with
overwhelming odds the more unique
antibodies your lymphocytes have the
more likely it is that one will
eventually find bind to and mark a
particular antigen once they've matured
B cells colonize or seed your secondary
lymphoid organs like your lymph nodes
and start roaming around in your blood
and lymph at this point they're still
naive and untested and they won't be
truly activated until they meet their
perfect enemy match which brings us back
to the flu virus and the right b-cell
finally bumps into an antigen it has
antibodies for usually in a lymph node
or in the spleen and recognizes it it
binds to it this someone is the full
power of the humoral immune response and
the cell basically goes into Berserker
mode once activated the B cell starts
cloning itself like crazy quickly
producing an army of similar cells all
with the instructions for the exact same
antibodies that are designed to fight
that one particular antigen most of
these clones become active fighters or
effector cells but if you become
long-lived memory cells that preserve
the genetic code for that specific
successful antibody this ensures that if
and when the antigen returns there will
be a prepared secondary immune response
that's both stronger and faster than the
first this is key to why vaccinations
are so brilliant and important which
I'll come back to in a minute but while
the memory cells are just there to hang
back and record things the effector or
plasma cells are packed with extra
amounts of rough endoplasmic reticulum
which acts as an antibody factory these
cells can mass-produce the same
antibodies over and over for that
particular invader spitting them out
into the humor at a rate of around 2,000
antibodies per second for four or five
days until they die and the antibodies
they make work the same way that the
membrane bound ones do they're just
free-floating so they ride the tides of
blood and lymph binding to all the
antigens they can find and marking them
for death now antibodies can't really do
the killing themselves but they do have
a few moves that could make it hard for
intruders to take hold what are their
most effective and common strategies is
neutralization where antibodies
physically block the binding sites on
viruses or bacterial toxins so they
can't hook up to your tissues and
because antibodies have more than one
binding site they can bind to multiple
antigens at the same time and a process
called agglutination the resulting
clumps can't get around easily which
makes it easier for macrophages to come
and gobble them up and not only that but
while all this is going on antibodies
are also ringing in chemical dinner-bell
calling in phagocytes from the innate
immune system and special lymphocytes
from the adaptive system to destroy
these messy little antigen antibody
clumps so the point of all of this in
the short term is to keep you healthy
but in the long term this process also
adds to your overall immunity the
humoral response allows your body to
achieve immunity by encountering
pathogens either randomly or on purpose
active humoral immunity is what we were
just talking about it's when b-cells
bump into antigens and start cranking
out antibodies this can occur naturally
like when you catch the flu or get
chickenpox or pick up some nasty
bacterial infection or it can happen
artificially particularly through
vaccination most vaccines are made of a
dead or extremely weakened pathogen and
they work on the premise that because
the secondary immune response is more
intense than a primary response by
introducing a pathogen into your body
you're priming it to fight
hard-and-fast should that antigen show
up again in the case of typically
non-fatal infections like the common flu
this immunity should at least spare you
from some of the most severe symptoms in
the case of more serious diseases like
polio smallpox measles and
whooping-cough vaccinations can be truly
life-saving now some antigens like those
from mumps or measles don't really
change much over time so a few
immunizations will leave you set for
life but others like influenza are
constantly evolving and changing their
surface antigens so immunity last year's
flu probably doesn't work against this
year's flu still acquired immunity
doesn't have to be active babies for
example naturally obtain passive humoral
immunity while still in the womb they
receive ready-made antibodies from their
mothers through the placenta and later
on through breast milk and that works
pretty well for a few months but the
protection is temporary because
passively obtained antibodies don't live
long in their new body and they can't
produce effector cells or memory cells
so a baby's own system won't remember an
antigen if it gets infected again you
can also acquire this kind of temporary
passive immunity artificially by
receiving exogenous antibodies from the
plasma of an immune donor this is what
recently saved some doctors and nurses
who had contracted the Ebola virus from
infected patients a serum was made from
the blood plasma of other medical
workers who had been infected and
survived the antibodies helped defend
the patients from the virus before their
own active immunity could identify that
particular antigen and start creating
their own antibodies if not the same as
a vaccine which immediately engages your
b-cells but it can buy a patient some
crucial life-saving time against an
infection that would otherwise quickly
kill but B cells and antibodies are only
part of the immunity equation there are
plenty of pathogens that quickly worm
their way right inside your cells where
they're safer from the humoral response
and free to multiply as much as they'd
like luckily your immune system has yet
another gameplan than a new set of
players ready to fight that final battle
with cell-to-cell combat make sure you
catch our final episode next week and
learn all about this epic battle royale
but as for today in our second to last
episode you learned how the adaptive
immune systems humoral response guards
your extracellular terrain against
pathogens we looked at how b-cells and
mature identify antigens and make
antibodies and how antibodies swarm
pathogens and mark them for death we
also talked about active and passive
humoral immunity and how vaccines work
thank you to our headmaster of learning
I evolved and thank you to all of our
patreon patrons if you are one of those
people I just thanks you make crash
course possible for the whole world and
also for yourself if you like crash
course and you want to help us make
videos like this one you can go to
patreon.com/crashcourse this episode was
filmed in the doctor Cheryl C Kinney
crash course studio it was written by
Kathleen Yale the script was edited by
Blake de pastino and our consultant is
dr. Brandon Jackson it was directed by
Nicholas Jenkins edited by Nicole
Sweeney our sound designer is Michael
Aranda and the graphics team is thought
cafe