Immune System, Part 1: Crash Course Anatomy & Physiology #45

Summary

Your body is constantly battling harmful microorganisms like staph, strep, and E. coli with its immune system, which consists of two key defenses: the innate (nonspecific) system and the adaptive (specific) system. The innate system acts quickly, using barriers like skin and mucous membranes, and internal defenses like phagocytes and natural killer cells. When pathogens breach these defenses, inflammation and fever kick in to combat invaders. This video focuses on the innate system’s role, highlighting how it uses various methods to protect you from everyday germs and injuries.

Takeaways

• 🛡️ Your body is constantly defending against microorganisms like staph, strep, and E. coli.
• 🧬 The immune system is a network of tissues, organs, and specialized cells that work together to protect the body.
• 💪 The innate (nonspecific) immune system is the first line of defense, using skin, mucous membranes, and internal defenses like phagocytes.
• 🎯 The adaptive (specific) immune system targets specific threats, learns from past infections, and prepares for future encounters.
• 🔥 Physical barriers like skin and mucous membranes provide effective protection from external threats.
• 🦠 Phagocytes, such as neutrophils and macrophages, engulf and destroy invading pathogens.
• 💥 Natural killer (NK) cells target infected or cancerous cells, triggering cell death through apoptosis.
• 🌡️ Inflammation, triggered by histamines, increases blood flow and helps fight infections through swelling, redness, and heat.
• 🤒 Fever, initiated by pyrogens, raises the body's temperature to enhance cell metabolism and restrict bacterial growth.
• 🔄 The immune response is a coordinated effort involving cells like neutrophils, macrophages, and lymphocytes to eliminate invaders and promote healing.

Q & A

What is the primary role of the immune system?

The immune system defends the body against harmful microorganisms, such as bacteria and viruses, to keep it healthy.

How does the innate immune system differ from the adaptive immune system?

The innate immune system is a nonspecific defense that immediately engages invaders, while the adaptive immune system is more targeted, taking longer to respond but designed to fight specific pathogens.

What are the two main physical barriers of the innate immune system?

The two main physical barriers are the skin and mucous membranes, which protect the body from external threats.

What happens when the skin or mucous membranes are breached?

When these barriers are breached, the second line of defense, such as phagocytes and antimicrobial proteins, is activated to attack invaders.

What role do neutrophils play in the immune system?

Neutrophils are a type of white blood cell that chase down and ingest pathogens. After performing this function, they often self-destruct, and their dead bodies contribute to the formation of pus.

How do macrophages differ from neutrophils?

Macrophages are larger, tougher immune cells that can engulf pathogens repeatedly, unlike neutrophils, which die after consuming pathogens.

What is the function of natural killer (NK) cells?

Natural killer cells patrol the blood and lymph, targeting and killing infected or cancerous cells by triggering apoptosis, or programmed cell death.

What triggers the inflammatory response in the immune system?

The inflammatory response is triggered by injuries, such as a cut, where histamines and other chemicals are released to cause vasodilation and attract immune cells to the site of infection.

What is the role of fever in the immune system's defense?

Fever is the body's systemic response to overwhelming infections. It increases the metabolism of cells for faster healing and deprives bacteria of essential nutrients like iron and zinc.

What is leukocytosis, and how does it assist the immune response?

Leukocytosis is the process where the body releases more neutrophils from bone marrow into the bloodstream to fight off an infection, especially during the inflammatory response.

Outlines

00:00

🦠 The Constant Battle for Your Health

Your body is constantly fending off various bacteria like staph, strep, and E. coli, which seek to use your body's resources for survival. To combat these invaders, your immune system employs a three-part defense system. Unlike other systems in your body, the immune system doesn't focus on a single organ or tissue but instead relies on a network of tissues, organs, and specialized cells. This system includes both innate (nonspecific) defenses and adaptive (specific) defenses. The innate system acts as the first line of defense, using external barriers like skin and mucous membranes, along with internal defenses like phagocytes and antimicrobial proteins, to protect you from invaders.

05:01

🛡️ The Elegance of Your Innate Defense System

The innate immune system, which includes physical and chemical barriers like skin, mucous membranes, and internal defense mechanisms, offers a robust, immediate response to infections. Your skin, acting as a strong barrier, prevents harmful microorganisms from entering the body. Mucous membranes, found in areas like the respiratory and digestive tracts, trap invaders. Chemical defenses such as stomach acid, mucus, and enzymes in saliva and tears destroy harmful bacteria. These simple yet effective methods provide crucial protection from daily exposure to pathogens.

🧬 Phagocytes and Natural Killer Cells: The Internal Defenders

When the first line of defense is breached, your body’s internal innate defenses activate. This includes fever, chemical signals, and inflammation. Phagocytes, including neutrophils and macrophages, rush to the scene. Neutrophils, which are abundant, act quickly but self-destruct after consuming pathogens, contributing to pus. Macrophages are larger, tougher, and can engulf multiple invaders. Natural killer cells, with their incredible ability to detect infected or cancerous cells by the absence of MHC1 proteins, induce apoptosis in faulty cells, ensuring a clean elimination of threats.

🔥 Inflammatory Response: The Body’s Alarm System

In response to injury or infection, the inflammatory response is triggered, sending chemical signals that cause redness, heat, swelling, and pain. Histamines released by mast cells help by dilating blood vessels, increasing permeability, and allowing more immune cells like phagocytes to reach the affected area. The swelling assists in blood clotting and healing, while macrophages and neutrophils clean up the damage. This coordinated effort ensures the body can quickly contain and eliminate pathogens while initiating healing processes.

🌡️ Fever: A System-Wide Defense

When local defenses are overwhelmed, the body triggers a fever, increasing the overall temperature to boost cell metabolism and healing. Fever also deprives pathogens of essential nutrients like iron and zinc by keeping them in the liver and spleen. This system-wide response allows the body to fight off large-scale infections more effectively. Despite the discomfort, fever serves as a critical part of the immune response, working alongside local defenses to eradicate invaders.

💪 Wrapping Up Innate Immunity

The innate immune system, from physical barriers to phagocytes and fever, plays a crucial role in defending against everyday infections and injuries. When these defenses are insufficient, the body calls in the adaptive immune system for more targeted responses. This comprehensive defense mechanism ensures that the body remains protected against a wide variety of threats. By understanding how the innate system works, we gain insight into the remarkable ways our bodies keep us healthy.

Keywords

💡Immune System

The immune system is the body’s defense mechanism against pathogens such as bacteria, viruses, and fungi. In the video, it is explained as a non-tissue-organ-specific system that involves various organs, tissues, and defense cells working together to protect the body. Its role in defending the body from constant threats is the core focus of the video.

💡Innate Defense System

The innate defense system, or nonspecific defense, is the body's immediate response to pathogens. It consists of physical barriers like skin and mucous membranes, as well as internal defenses such as phagocytes and antimicrobial proteins. The video explains it as the first line of defense, always ready to engage threats without needing prior recognition of specific pathogens.

💡Adaptive Defense System

The adaptive, or specific, defense system is a specialized immune response that targets specific pathogens. Unlike the innate system, it takes longer to activate but retains memory of pathogens for faster responses in the future. In the video, it’s compared to special forces like 'Seal Team Six,' showcasing its precision in eliminating specific threats.

💡Phagocytes

Phagocytes are immune cells that engulf and digest foreign invaders like bacteria. The video describes different types of phagocytes, such as neutrophils and macrophages, that act like Pac-Man, consuming harmful pathogens. These cells are part of the innate immune system and provide the first response to infections.

💡Neutrophils

Neutrophils are the most abundant type of white blood cell in the body and are part of the innate immune system. They self-destruct after attacking pathogens, forming pus as seen in infections. The video highlights their role as the first wave of defense cells that engage invaders before dying off to be replaced by macrophages.

💡Macrophages

Macrophages are large phagocytes derived from monocytes that can continuously devour pathogens and dead cells. In the video, they are portrayed as tougher and more durable than neutrophils, capable of engulfing pathogens multiple times. They play a key role in cleaning up after neutrophils in an immune response.

💡Natural Killer Cells

Natural killer (NK) cells are part of the innate immune system, responsible for identifying and destroying infected or cancerous cells. They do this by recognizing the absence of MHC1 proteins on the surface of infected cells. The video emphasizes their unique ability to induce apoptosis in defective cells, making them highly effective against internal threats.

💡Inflammatory Response

The inflammatory response is the body's reaction to injury or infection, characterized by redness, heat, swelling, and pain. The video describes how chemical signals like histamine trigger vasodilation and increased blood vessel permeability, allowing immune cells to reach the site of injury and begin repair. This response is crucial for healing and containing infections.

💡Histamine

Histamine is a chemical released by mast cells during an immune response that causes vasodilation and increases blood vessel permeability. In the video, histamine is described as a key player in the inflammatory response, helping immune cells like phagocytes access the site of infection or injury, leading to redness and swelling.

💡Fever

Fever is a systemic immune response triggered by pyrogen chemicals that raise the body's temperature to fight off infection. The video explains that fever enhances the metabolic rate of cells to speed up healing and restricts bacterial growth by limiting the availability of iron and zinc. It is portrayed as a tool used when local immune defenses are overwhelmed.

Highlights

Your body is constantly engaged in a battle against harmful microorganisms like staph, strep, and E. coli.

The immune system consists of various tissues, organ systems, and widely-distributed defense cells that work together to protect the body.

The innate immune system acts as the body's first line of defense, using physical barriers like skin and mucous membranes, along with internal defenses like phagocytes and antimicrobial proteins.

The adaptive immune system is a specialized response that takes longer to activate but targets specific pathogens and remembers them for future attacks.

Skin serves as a highly effective physical barrier to keep harmful microorganisms out, and mucous membranes in body cavities offer similar protection.

The body’s chemical defenses include stomach acid, mucus, saliva, and enzymes that help fight off bacteria and viruses.

Phagocytes like neutrophils and macrophages play key roles in attacking and consuming pathogens.

Neutrophils are the most abundant type of white blood cells, and they self-destruct after devouring pathogens.

Macrophages are larger phagocytes that can consume multiple pathogens and play a longer-lasting role in the immune response.

Natural killer (NK) cells patrol the blood and lymph, targeting and killing infected or cancerous cells by triggering apoptosis.

When the body is injured, the inflammatory response is activated, releasing histamines that cause redness, swelling, and heat to help with healing.

The inflammatory response attracts phagocytes to the injury site, allowing them to fight off pathogens and clean up dead cells.

Neutrophils are the first to arrive at the injury site, but as they die off, macrophages replace them to continue the defense.

When overwhelmed by a serious infection, the body raises its temperature (fever) to increase metabolic rates and slow down pathogen growth.

Fever also prompts the liver and spleen to hold onto iron and zinc, which bacteria need to grow, further hampering the infection.

Transcripts

You may not know it, but your body is engaged in a never-ending battle.

You are literally covered in staph and strep and e coli, and all sorts of dubious characters

that are intent on using you, and your body’s many resources, to feed themselves, find shelter,

and reproduce as much as they want.

And, hey, we all gotta make a living. But it is not your job to give these guys a free lunch.

So your body has developed a three-part policy toward these shady customers, and its enforcement

is handled by your immune system.

The immune system is different from all the other systems we’ve talked about this year

in that it’s not a specific, tissue-organ-system kind of system.

Instead, it involves a whole bunch of different tissue groups, organ systems, and

specialized-but-widely-distributed defense cells.

Together, this league of extraordinary substances joins forces to perform all of the defense

functions your body depends on to keep you alive in an incredibly germy world.

And the first line of defense in this never-ending battle? That’s your innate, or nonspecific, defense system.

Like your average frontline soldier, it’s prepared to immediately engage with anyone

suspicious, and it mostly includes stuff we were born with, like the external barricades

of your skin and mucous membranes, and internal defenses like phagocytes, antimicrobial proteins,

and other attack cells.

But some enemies must be fought with special forces. And here, your body can deploy your

adaptive, or specific defense system, which is more like your Seal Team Six.

It takes more time to call in, but it’s specially designed to go after specific targets.

And it keeps files on those bad guys so it knows how to handle them next time around.

But today we’re going to focus on your innate system, and look at how it uses an arsenal

of physical and chemical barriers, killer cells, and even fever, to keep you healthy.

Proving that sometimes, the symptoms we associate with illness are actually the signs that we're healing.

Just because something is simple doesn’t mean that it can’t be elegant.

I mean, your body is capable of some incredibly sophisticated things, including

defending itself from infection.

But occasionally there’s something to be said for brute force.

And a lot of your innate immune system’s functions aren’t exactly subtle. For example,

your body’s very first line of defense is a simple physical barrier. And it works!

Like a wall around a fortress, your skin does a fantastic job of keeping out all manner

of malevolent microorganisms.

As long as that tough, keratinized epithelial membrane doesn’t get torn open or busted

up too much, you could probably, like, make snowballs out of raw sewage and still be alright.

Although...no. No.

Your many mucous membranes also provide a handy physical barrier. You’ll remember

that they line any cavity that opens up into the germy outside world, including the respiratory,

digestive, urinary, and reproductive tracts.

Not only do your skin and mucosa supply simple physical protection,

they also pack some serious chemical weaponry.

Eat some questionable leftovers for lunch? Don’t worry, your stomach is literally filled

with acid, so you probably are covered.

Walk face-first into your co-worker’s nasty sneeze cloud? No worries, your nasal passages

can whip up a tissue-box worth of sticky mucus to help trap viruses before they enter your lungs.

You’ve also got bacteria-fighting enzymes in your saliva and lacrimal eye fluid, and

peptides called defensins in your skin and membranes that help keep bacteria and fungi

from setting up shop around inflamed or scraped skin.

Which, no matter how careful you are, you’re gonna get, one way or another.

Maybe you shave with a dull blade. Or you just brush your teeth too hard. And DON’T

GET ME STARTED about the dangers of bagel-cutting.

So when you’ve breached that first, simple line of defense, it’s time to call on your

second line of internal innate defenses.

This is where your body starts pulling strategic maneuvers like firing up a fever, releasing

chemical signals, causing inflammation, or other defensive tactics that help identify

and attack infectious invaders.

Some of the first defensive cells on the scene are your phagocytes. Their name literally

means “to eat,” and like Pac-Man, they indiscriminately chase down intruders and

gobble them up. And they come in a few different varieties:

First you’ve got neutrophils, which are the most abundant type of your white blood

cells. They kind of self-destruct after devouring a pathogen. And, in fact, you’ve actually

seen piles of their little dead bodies, because that’s what pus is made of.

But the bigger, tougher phagocytes are the macrophages. They’re derived from monocyte

white-blood cells that have moved out of the blood stream to occupy tissues. And some are

free types that patrol tissues looking for creepers, while others are fixed -- attached

to fibers in specific organs, devouring anything suspicious that passes by.

So when a macrophage in, say, the finger I just cut slicing a bagel, sees a new bacterium

coming along, it snares it using cytoplasmic extensions, reels in it, completely engulfs

it, and -- essentially -- digests it and spits the rest out.

And unlike neutrophils, it can do this over and over again, like a boss.

But not all your defense cells are phagocytic. You’ve also got cells with what is by far

the awesomest name of any cell in the body: the natural killer cells. You can call them

NK cells if you want to, but like, why would you do that?

Anyway, these tiny assassins patrol your blood and lymph looking for abnormal cells, and

are unique in that they can kill your own cells if they are infected with viruses or

have become cancerous.

How can they tell?

A normal, healthy cell contains a special protein on its surface called MHC1, or major

Histocompatibility Complex. But if it’s infected, it stops making that protein.

And if an NK cell detects a defective cell, it doesn’t swallow it whole like a macrophage

-- it pokes it with an enzyme that triggers apoptosis, or programmed cell death, which

is pretty awesome.

So those are some ways your innate immune cells handle their enemies, but how do they

know where to look in the first place?

So, let’s talk strategery.

So, say you’re in a banana factory and you slip on a banana peel and scrape your knees.

Your outer fortress has been breached, and the pathogens are just flooding in like orcs

through Helm’s Deep.

Banana factories are very dirty places.

Now your body wants to contain the spread of pathogens, clean up the mess, and get healing

as quickly as possible, so it cues up your inflammatory response.

This is basically an internal fire alarm, only it uses chemicals instead of sirens to

get the message across, and instead of smoke and fire you sense redness, swelling, heat,

and pain.

For example, in the event of injury, specialized mast cells in your connective tissue send

out histamine molecules.

And histamine is great at calling in the cavalry.

For one thing, it causes vasodilation, which creates redness and heat at the site of the

injury. Now, those things might freak you out a little, but they’re actually signs

of healing -- the increased temperature, for example, ratchets up the cells’ metabolic

rates so they can repair themselves faster.

Meanwhile, histamines and other inflammatory chemicals also increase the permeability of

blood vessels, causing nearby capillaries to release protein-rich fluids.

This causes swelling -- which again, is actually a good thing -- because that leaked protein

helps clot blood and form scabs, while the lymphatic system sucks up and filters that

extra fluid, cleaning it up before putting it back into your bloodstream.

And of course, like chum to sharks, an inflamed knee is also going to attract a bunch of local

phagocytes -- which find it easier to escape your now-leaky capillaries -- and lymphocytes

that are also flowing freely, helping to destroy pathogens while also cleaning up dead-cell

wreckage.

And don’t forget: During all this, the neutrophils have been doing their best, but they were

the first wave to arrive, so by this time, they’re starting to die in heaps.

They’re triggered when the injured knee-skin cells release chemicals that begin leukocytosis

– the release of neutrophils from the bone marrow where they’re made into the bloodstream.

To attract the neutrophils to the damaged area, inflamed endothelial cells in the capillaries

send out chemicals that act like homing devices-- and when the neutrophils arrive, they cling

to the capillary walls near the injury, flatten themselves out and squeeze through the vessel

walls to get to work.

Your big monocytes eventually roll up to the battle, and transform into hungry macrophages,

replacing that first line of now-dead neutrophils and basically just eating up any lingering

enemies and then cleaning up the carnage.

Now, all this works pretty well in most circumstances. But you may have noticed if you’ve sustained

a more major injury, or are battling an especially nasty virus or infection, that sometimes your

local troops get overrun.

When white blood cells and macrophages run into more foreign invaders than they can handle,

they let loose pyrogen chemicals that tap the hypothalamus and raise your body’s thermostat,

calling in a systemic fever to burn everything.

The resulting temperature rise increases the metabolism of your cells so they can heal

faster, and it also tells the liver and spleen to hold onto all of their iron and zinc, so

those things can’t contribute to bacterial growth.

But even then, sometimes, well sometimes you find yourself facing a more formidable foe.

That’s when you call in the specialists -- your adaptive immune defenses.

And to learn exactly how they save the day, you have to watch next time.

But for now you learned that your immune system’s responses begin with physical barriers like

skin and mucous membranes, and when they’re not enough, there are your phagocytes -- the

neutrophils and macrophages. You also learned about natural killer cells and the inflammatory

response, and watched as all of these elements saved the day when you slipped on a banana

peel.

Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon

patrons whose monthly contributions make Crash Course possible, not only for themselves,

but for everybody. If you like Crash Course and you want to help us keep making videos

like this, 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.