In biology we use something called the hierarchy of complexity to show how things are made
up of smaller things.
Organ systems are made of organs which are made of tissues which are made of cells and
so on.
Organ systems like the skeletal system are easy to visualize.
The entire skeleton is made of individual bones.
Each bone is made of different kinds of bone tissue, and we can get more granular with
individual cell types.
But the immune system is a little harder to picture.
Usually when people teach the immune system, they talk about the immune response and identify
different cell types like B cells or macrophages.And there’s nothing wrong with that, but we
usually gloss over one of the biggest parts of systems level anatomy — organs.
So in today’s video, I’m going to teach you the organs of the immune system.
Hello and welcome.
If you’re new here, my name is Patrick and this channel is all about anatomy and physiology.
One of the things we’ll notice as we explore the immune system organs is how rare it is
for an organ to be exclusively part of the immune system.
Like our skin is classified as part of the integumentary system, but it also serves as
a barrier against pathogens, same thing with our mucous membranes.
Likewise, our stomachs kill germs with stomach acid, but we always think of the stomach as
a digestive system organ.
And the biggest elephant in the room is how the immune system is so tied up with the lymphatic
system that calling something exclusively an immune organ is impossible.
In reality, plenty of organs pull double duty and could easily be considered part of multiple
systems.
Regardless, there are some organs we can focus on right away.
Let’s start with the spleen, the biggest organ in your immune system.
This thing is found in your upper left abdominal quadrant, posterior to your stomach and lateral
to your left kidney.
One side butts up against the diaphragm while the other side, the visceral side, faces the
other organs and it’s where you find the spleen’s blood supply.
If we were to cut this thing open, it kind of looks like the classic kidney cross section,
and there is some overlap.
That area where the splenic artery and vein exit the spleen is called the hilum, which
is the same name given to the area on the kidney where blood vessels enter and exit.
Speaking of which, the spleen handles a lot of blood.
Its purple coloring actually comes from having so many blood vessels.
You may have heard that red blood cells only live for about 4 months before dying off,
and it’s actually the spleen that acts like a filter that separates the old cells from
the young ones.
When it detects a cell past its prime, it filters it out of the blood stream and destroys
it through phagocytosis.
Then it recycles some of the molecular components like hemoglobin so they can be used when making
new cells.
Now the bulk of the spleen is made up of what’s called pulp, and there are two types: red
pulp and white pulp.
Unfortunately for anatomy students, because of how we have to stain them and look at them
under the microscope, the white pulp looks more like purple nodes while the red pulp
looks mostly pink.
White pulp is pretty much a clump of lymphocytes stacked around a blood vessel, so the white
pulp is where the majority of the white blood cells live in the spleen.
In particular, B and T cells can use this space to mature into fully functioning cells
or to create antibodies.
Meanwhile, red pulp is made of splenic sinusoids, which are basically wide blood vessels, plus
some connective tissue.
This is where the majority of blood filtration happens, but it’s also used to store red
blood cells, white blood cells, and platelets just in case the body needs them.
Also quick fun fact, the same Gray who wrote the original Gray’s Anatomy textbook initially
became popular because he wrote an enormous 300 page treatise about the spleen, the picture
of which I’m showing you right here.
You can learn more about Gray on my history of medicine channel here.
Let’s move on to the bone marrow, which is where lots of immune cells are born.
These cells have complicated lifestyles, but this is where they start.
There are two types of bone marrow: red and yellow, and they have different jobs.
Yellow marrow is found in the middle of long bones like the femur and humerus, and it’s
mostly fat cells.
Although, yellow marrow does have some stem cells that can become things like cartilage
or bone cells.
We’re more interested in red bone marrow, which we mostly find in the spongy ends of
long bones, but also in bones like the scapula, vertebrae, and pelvis.
Red marrow is where you find hematopoietic stem cells, or HSCs, which things turn into
the different blood cells through a process called hematopoiesis.
That’s easily a video by itself, but here are the final products.
You’ll find granulocytes, which are immune cells that secrete little chemicals called
granules.
This includes basophils, neutrophils, and eosinophils, which you can see in this blood
smear.
You’ll also find monocytes, which become macrophages — big cells that engulf pathogens
through phagocytosis.
You’ll find my favorite type of cell, the megakaryocyte, which makes the platelets that
clot injuries.
You’ll also find erythrocytes, or red blood cells.
Finally, you’ll see lymphocytes, which are white blood cells.
There are about 2 trillion lymphocytes in your body, and together, those cells weigh
about as much as your brain or liver.
And some of these cells migrate to our next immune organ, the thymus, to turn into mature
T-lymphocytes.
I want to clarify something real quick: the thymus is different from the thyroid.
The thyroid is a gland superior to your trachea that’s mostly responsible for pumping out
hormones, which makes it part of the endocrine system.
The thymus is inferior to that, deep to the sternum — it spans from the manubrium, that
top portion of your sternum, and runs down a few inches to around rib number 4.
It’s located anterior to the great blood vessels of the heart like the aorta and has
two distinct lobes.
The organ itself is covered in connective tissue and has two main zones, the darker
red and more peripheral cortex and the lighter, deeper medulla.
Just like in the kidney, the cortex is where the action happens.
The main purpose of the thymus is to be kind of like an incubator for T cells.
Immature T cells called thymocytes come from the bone marrow, float through the bloodstream,
then enter the thymus.
You’ll find most of the thymocytes in cortex of the thymus, but one of the processes that
turns thymocytes into mature T cells happens in the medulla.
It’s a process called negative selection.
Then, in between the cortex and medulla is the corticomedullary junction where you find
the most mature T cells plus some other immune cells and lots of blood vessels.
Once those T cells mature, they can enter circulation again and be part of the immune
response.
Before we move on, another fun history fact, the T cell got its name from being a thymus-derived
immune cell.
Moving on from the spleen, we’ve got a complex network of lymphatic vessels and lymph nodes
that work together with the immune system.
And this is where that question of “what counts as the immune system?” gets a little
murky.
The lymphatic system forms a distinct network around the body, a lot like the vascular system,
so you can totally say that the lymphatic system is a separate, traditional organ system.
But its function is so connected to the immune system that it belongs in this video.
For our purposes, we’ll include the lymphatic vessels, lymph nodes and a few special lymphatic
clusters in our discussion.
So, high level, you can think of the lymphatic vessels like a sewage system.
The lymphatic vessels collect fluid that leaks out from the circulatory system and picks
up stray proteins and pathogens along the way.
That fluid is then called lymph and it’ll eventually be returned to the circulatory
system, but only after it's cleaned.
The lymphatic vessels send the fluid to lymph nodes, which are little bean shaped tissues
packed with immune cells like T and B cells, macrophages, and dendritic cells.
These things not only filter out pathogens, but they make up part of the adaptive immune
response.
When those cells in the lymph nodes come across a germ, they identify the germ’s antigens,
the proteins that identify the germ as a bad guy, so that they can make antibodies against
it and destroy it during future exposures.
Meanwhile, there are lymphoid organs that have lymphatic tissue, but aren’t lymph
nodes.
For instance, there are clumps of lymphoid nodules around certain high risk areas in
your body.
Like the digestive system can come in contact with a bunch of pathogens through the food
and drinks we consume.
So you’ll find clusters of lymphatic tissue around the esophagus and intestines called
the GALT, or gut associated lymphatic tissue.
Then there are clumps of these tissues in the small intestine called Peyer's patches,
which so big that they’re visible with the naked eye.
But the best known lymphoid organs are probably the tonsils, which contain white blood cells,
and attack germs before they can enter the digestive system.
The last organ of the immune system that I’ll include here is the Appendix.
This guy is a tiny, worm-like projection hanging off the large intestine.
And the fact it looks like a worm gives it its Latin name — Appendix virmiformis.
Vermi is the root word for worm, just like vermicelli pasta.
You’ll find the appendix really close to a structure called the ileocecal valve, the
sphincter muscle between your ileum, the final portion of your small intestine, and your
cecum, the first part of your large intestine.
The appendix is usually around 9 centimeters long, although the longest on record is 35
centimeters.
I have no idea where you even put that much appendix.
When I was a kid, scientists weren’t totally sure what the appendix did.
It was fairly common to see it surgically removed since it could get infected.
But these days, we know a lot more about it.
The appendix is basically a tube with a few different layers to it.
It has some lymphatic tissue that’s involved with maturing B lymphocytes and producing
antibodies.
But it’s also home to a bunch of healthy gut bacteria.
That means the appendix can repopulate the large intestine if you get severe diarrhea
and wipe out the existing microbiome.
Also that is a purposeful juxtaposition of diarrhea and wipe.
It’s called comedy, look it up.
Before you go, I think you’ll like this video I made about how the kidneys work, and
this one I made about the liver.
They’re simple introductions to complicated topics, and I hope they’ll make studying
those organs less intimidating.
As always, thank you to my patrons on Patreon.
If you’re in the financial position to support the channel, I appreciate the help.
I also set up a PayPal for the channel if you want to buy me coffee for one time only.
Have fun, be good, thanks for watching.