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Crash Course: Anatomy & Physiology
Innate Immune Systems
00:00 - 06:40
Hank explains innate immune systems and how the signs that we may associate with sickness are actually be symptoms of your immune system working.
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Video Transcript
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You may not know it, but your body is engaged in a never-ending battle.
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You are literally covered in staph and strep and e coli, and all sorts of dubious characters
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that are intent on using you, and your body’s many resources, to feed themselves, find shelter,
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and reproduce as much as they want.
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And, hey, we all gotta make a living. But it is not your job to give these guys a free lunch.
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So your body has developed a three-part policy toward these shady customers, and its enforcement
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is handled by your immune system.
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The immune system is different from all the other systems we’ve talked about this year
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in that it’s not a specific, tissue-organ-system kind of system.
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Instead, it involves a whole bunch of different tissue groups, organ systems, and
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specialized-but-widely-distributed defense cells.
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Together, this league of extraordinary substances joins forces to perform all of the defense
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functions your body depends on to keep you alive in an incredibly germy world.
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And the first line of defense in this never-ending battle? That’s your innate, or nonspecific, defense system.
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Like your average frontline soldier, it’s prepared to immediately engage with anyone
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suspicious, and it mostly includes stuff we were born with, like the external barricades
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of your skin and mucous membranes, and internal defenses like phagocytes, antimicrobial proteins,
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and other attack cells.
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But some enemies must be fought with special forces. And here, your body can deploy your
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adaptive, or specific defense system, which is more like your Seal Team Six.
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It takes more time to call in, but it’s specially designed to go after specific targets.
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And it keeps files on those bad guys so it knows how to handle them next time around.
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But today we’re going to focus on your innate system, and look at how it uses an arsenal
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of physical and chemical barriers, killer cells, and even fever, to keep you healthy.
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Proving that sometimes, the symptoms we associate with illness are actually the signs that we're healing.
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Just because something is simple doesn’t mean that it can’t be elegant.
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I mean, your body is capable of some incredibly sophisticated things, including
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defending itself from infection.
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But occasionally there’s something to be said for brute force.
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And a lot of your innate immune system’s functions aren’t exactly subtle. For example,
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your body’s very first line of defense is a simple physical barrier. And it works!
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Like a wall around a fortress, your skin does a fantastic job of keeping out all manner
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of malevolent microorganisms.
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As long as that tough, keratinized epithelial membrane doesn’t get torn open or busted
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up too much, you could probably, like, make snowballs out of raw sewage and still be alright.
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Although...no. No.
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Your many mucous membranes also provide a handy physical barrier. You’ll remember
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that they line any cavity that opens up into the germy outside world, including the respiratory,
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digestive, urinary, and reproductive tracts.
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Not only do your skin and mucosa supply simple physical protection,
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they also pack some serious chemical weaponry.
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Eat some questionable leftovers for lunch? Don’t worry, your stomach is literally filled
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with acid, so you probably are covered.
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Walk face-first into your co-worker’s nasty sneeze cloud? No worries, your nasal passages
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can whip up a tissue-box worth of sticky mucus to help trap viruses before they enter your lungs.
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You’ve also got bacteria-fighting enzymes in your saliva and lacrimal eye fluid, and
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peptides called defensins in your skin and membranes that help keep bacteria and fungi
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from setting up shop around inflamed or scraped skin.
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Which, no matter how careful you are, you’re gonna get, one way or another.
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Maybe you shave with a dull blade. Or you just brush your teeth too hard. And DON’T
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GET ME STARTED about the dangers of bagel-cutting.
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So when you’ve breached that first, simple line of defense, it’s time to call on your
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second line of internal innate defenses.
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This is where your body starts pulling strategic maneuvers like firing up a fever, releasing
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chemical signals, causing inflammation, or other defensive tactics that help identify
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and attack infectious invaders.
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Some of the first defensive cells on the scene are your phagocytes. Their name literally
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means “to eat,” and like Pac-Man, they indiscriminately chase down intruders and
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gobble them up. And they come in a few different varieties:
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First you’ve got neutrophils, which are the most abundant type of your white blood
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cells. They kind of self-destruct after devouring a pathogen. And, in fact, you’ve actually
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seen piles of their little dead bodies, because that’s what pus is made of.
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But the bigger, tougher phagocytes are the macrophages. They’re derived from monocyte
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white-blood cells that have moved out of the blood stream to occupy tissues. And some are
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free types that patrol tissues looking for creepers, while others are fixed -- attached
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to fibers in specific organs, devouring anything suspicious that passes by.
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So when a macrophage in, say, the finger I just cut slicing a bagel, sees a new bacterium
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coming along, it snares it using cytoplasmic extensions, reels in it, completely engulfs
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it, and -- essentially -- digests it and spits the rest out.
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And unlike neutrophils, it can do this over and over again, like a boss.
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But not all your defense cells are phagocytic. You’ve also got cells with what is by far
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the awesomest name of any cell in the body: the natural killer cells. You can call them
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NK cells if you want to, but like, why would you do that?
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Anyway, these tiny assassins patrol your blood and lymph looking for abnormal cells, and
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are unique in that they can kill your own cells if they are infected with viruses or
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have become cancerous.
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How can they tell?
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A normal, healthy cell contains a special protein on its surface called MHC1, or major
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Histocompatibility Complex. But if it’s infected, it stops making that protein.
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And if an NK cell detects a defective cell, it doesn’t swallow it whole like a macrophage
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-- it pokes it with an enzyme that triggers apoptosis, or programmed cell death, which
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is pretty awesome.
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So those are some ways your innate immune cells handle their enemies, but how do they
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know where to look in the first place?
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So, let’s talk strategery.
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So, say you’re in a banana factory and you slip on a banana peel and scrape your knees.
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Your outer fortress has been breached, and the pathogens are just flooding in like orcs
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through Helm’s Deep.
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Banana factories are very dirty places.
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Now your body wants to contain the spread of pathogens, clean up the mess, and get healing
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as quickly as possible, so it cues up your inflammatory response.
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This is basically an internal fire alarm, only it uses chemicals instead of sirens to
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get the message across, and instead of smoke and fire you sense redness, swelling, heat,
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and pain.
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For example, in the event of injury, specialized mast cells in your connective tissue send
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out histamine molecules.
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And histamine is great at calling in the cavalry.
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For one thing, it causes vasodilation, which creates redness and heat at the site of the
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injury. Now, those things might freak you out a little, but they’re actually signs
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of healing -- the increased temperature, for example, ratchets up the cells’ metabolic
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rates so they can repair themselves faster.
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Meanwhile, histamines and other inflammatory chemicals also increase the permeability of
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blood vessels, causing nearby capillaries to release protein-rich fluids.
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This causes swelling -- which again, is actually a good thing -- because that leaked protein
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helps clot blood and form scabs, while the lymphatic system sucks up and filters that
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extra fluid, cleaning it up before putting it back into your bloodstream.
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And of course, like chum to sharks, an inflamed knee is also going to attract a bunch of local
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phagocytes -- which find it easier to escape your now-leaky capillaries -- and lymphocytes
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that are also flowing freely, helping to destroy pathogens while also cleaning up dead-cell
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wreckage.
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And don’t forget: During all this, the neutrophils have been doing their best, but they were
