Picture this: you wake up to the blissful sound of birds chirping—because you forgot to set an   alarm.

You take the quickest shower of your life,  throw on some clothes, and… that’s the bus. So, you hurry outside with your wet hair,  and you hear someone call after you,   “Put a hat on!

You’ll catch a cold!” Well, it turns out… that’s actually a  myth. The good news is having wet hair when it’s cold outside doesn’t cause contagious illness.   The only things that do are pathogens, which can be viruses or microorganisms, like bacteria.  The not-so-great news? Potential pathogens are everywhere…and it is true that we’re   more susceptible to them in cold weather.

But thankfully, animals have an Avengers-style team   of defenses against pathogens, from snot and oily skin to cells that never forget a face…or, virus.  Hi! I’m Dr. Sammy, your friendly  neighborhood entomologist,   and this is Crash Course Biology.

Now,  get ready for some infectious theme music.  [THEME MUSIC] When you think about fighting infection,  the first thing that comes to mind probably   isn’t skin. But skin and the things that  grow out of it — hair, nails, feathers,   and scales — are the first layer of an animal’s  built-in defense system or its innate defenses.   ‘Cause you know, awl’ of dis isn’t just for looks! We call this outer layer the integumentary system.   It’s one of the ways the body prevents sickness  and injury—basically, it keeps your insides in,   and the outside world mostly out.

In general, skin and its associates protect   animals from injury and block pathogens  that can cause disease. Hair, feathers,   and scales can also stop disease-carrying  parasites in their tracks. Think about how much   harder it’d be for a mosquito to bite you if you  were covered in a thick layer of scales!

Instead,   humans had to invent cargo pants and bug spray. But although we’re basically the naked mole rats   of primates, our skin is still a powerful  barrier against pathogens. And if you get a   paper cut or scratch up your knee, your skin is  a pro at regrowing cells and fixing itself up.  And skin isn’t just a physical forcefield.  The skin of many animals produces an acidic,   oily mixture called sebum that helps kill  pathogens.

Sebum also makes your hair and skin   greasy…but hey, at least it’s serving a purpose  besides making you break out on picture day.  So, our bodies are great at protecting us from the gnarly stuff outside,   whether we remember our hats or not. But what happens when that stuff finds its way inside? Because most animals have to breathe and eat   stuff from their environment, we need openings  like mouths and noses that allow stuff into   our digestive and respiratory tracts.

But those openings can also sometimes   let in harmful things. Like food that’s been  infected with bacteria or air containing virus   particles, like the flu. Thankfully,  evolution has prepared us for this.  Any part of an animal’s body that’s open to the  outside world has protective barriers.

For those   of us with backbones, AKA vertebrates, it’s  mostly a layer of mucus that contains immune   cells and natural antibiotics. And while snot gets a bad rap,   especially when you have to blow your nose in  the middle of a perfectly silent classroom,   it’s doing you a service. That nose goop acts like  glue, trapping pathogens before they can sneak   deeper into your body.

That’s partly why you get  so snotty when you’re sick: more mucus traps more   germs, which means microbes and viruses get blown  or coughed out of your body just a little faster.  Still, if you’ve ever gotten sick…which I assume  you have, if you’re not a robot. Unless…? No,   I’ll assume you’ve caught a cold at some point.  And if you have, you’ve discovered that pathogens   can still sneak past this mucus barrier.

But that doesn’t mean they’re home   free. This is where the immune system  really steps into high gear, and where   things get really cool. Or really sweaty.

When a pathogen goes deeper into the body,   an animal’s innate defenses attack  unwanted cells and make it harder   for pathogens to survive and reproduce. Take scavenger cells, a specific type   of white blood cell. A scavenger cell can  swoop in, surround a pathogen, and eat it,   like Pacman.

And then, it spits out what’s  left of the pathogen so other parts of the   immune system can see what had the AUDACITY,  the unmitigated gall to sneak into this body.  And then there are natural  killer cells. Seriously,   that’s the scientific name. They can take down  microbial infections and cancerous tumors by   siccing a sort of poison molecule on them.

Innate defenses also include inflammation,   where an area of the body becomes hot and swollen  in response to an irritant. Like, when you get   stung by a bee, the site of the sting gets red and  swells with nutrient-rich blood that’s carrying   immune cells to fight back against the bee venom. And when you get a fever, this inflammatory   response happens on the whole-body level.  It sounds weird, but think about it:   pathogens are infecting you — or a vulture, or a  whale, or your hamster — because you seem like a   pretty sweet apartment.

You’re warm, you’ve got  running water and energy, and your body seems   like a nice place to set up shop and multiply. When your body detects this new unwelcome tenant,   it cranks up the thermostat until it’s  obnoxiously hot, giving you a fever,   and making the pathogens so uncomfy that they  grow more slowly or die. Your body would rather   have you stuck in bed feeling both sweaty and  freezing than have that invader stick around.  That means when we or other animals feel sick,  it’s often not the pathogen’s fault — at least,   not directly.

The symptoms that specific pathogens  cause aren’t usually things you can feel — at   least not right away. It’s how our bodies respond  to pathogens that we experience as being “sick.”  Now, on some occasions, pathogens can   successfully fly under the radar. They’ve made it  inside and started reproducing, but the body shows   no symptoms at all.

It might be launching some  low-level defenses, but they’re minimal enough   that the person infected doesn’t even realize  it. We call this an asymptomatic infection.  These came into the limelight when  the COVID-19 pandemic started. You   might have heard of people having asymptomatic  infections, which meant they could unknowingly   infect others with COVID.

But you can have an  asymptomatic infection of many other illnesses   as well. A number of sexually-transmitted  infections are a prime example of this.  Finally, it’s worth noting that innate   defenses aren’t /just/ for protecting an animal  against illness. They’re also involved in healing.  For example, inflammation can send a lot of blood  to a wounded area, which provides nutrients for   cells that need to quickly duplicate.

All that  blood also carries discs called platelets that are   involved in blood clotting — where the body makes  a barrier over a wound to stop bleeding. You’ll   recognize the end result as a scab. Even the pain  part of inflammation can be an important part of   healing because it makes us protect the injury.

So yeah, our innate defenses do a lot to  keep us safe and healthy. But vertebrates   have yet another defensive layer, one with an  even sharper eye for the pathogens that could   wreak havoc in our bodies: adaptive immunity. While the gist of innate immunity is “Hey,   there’s an invader in here, let’s get ‘em,”  adaptive immunity is about recognizing and   remembering specific pathogens.

So, it’s not  just “an invader,” it’s “Bob the flu virus is   back again; we know how to deal with this guy.” Understanding this system has allowed scientists   to develop one of the world’s  most life-saving technologies:   vaccines. Let’s head to the Thought Bubble…. Welcome to my adaptive immune system.

This   is me getting my first COVID-19  vaccine. You got this, buddy!  Now, my body will start making proteins:  the same one on the real COVID-19 virus.   This protein, called an antigen,  is basically a cellular name tag.  Now, here come the super important white  blood cells to scope out the situation—hey,   T cells. They’ve never met these proteins before,  and they’re not fans.

They call in other parts   of the immune system to wipe them out. First up: antibodies. These custom-made   molecules flag down specific pathogens.

They stick  themselves to the virus to keep it from causing   harm and then, this signals reinforcements. Next, we’ve got memory cells — so-called   because they sort of “remember” what those  proteins look like so they can attack the   virus again if needed. They do this by making  receptors that fit into just one type of antigen,   like one of those two-part friendship necklaces.

This whole process has prepped my body for a   potential encounter with the real COVID-19 virus.  So, when I’m exposed to it a couple months later,   my immune system springs into action more quickly. Memory cells bind to the antigen and quickly   trigger a response. The antibodies then swoop  in to smother the virus, and the immune cells   get to work, too.

Because my body has practiced  this exact mission before, it can respond much   faster, and I’ll be COVID-free in no time. Thanks, Thought Bubble! In this scenario,   you can substitute the COVID-19 virus with  basically any bacteria, virus, or other   pathogen that could infect an animal.

Like, if  you’ve ever gotten chickenpox or strep throat,   you likely have antibodies and memory cells  for those pathogens in your body right now.  But you may be wondering, where exactly /is/   the immune system? Like, if the integumentary part  is outside, sort of—on my skin, and in my snot and   stuff—where inside me do the other parts happen? And the answer is… kind of everywhere.

Last   episode, we talked about the lymphatic system as  part of the body’s infrastructure — a series of   vessels and tubes throughout your body  that help keep your fluids balanced.  But, the lymphatic system is also a home base for  a lot of adaptive immune cells. They hang out in   bean-shaped structures called lymph nodes and  screen everything that comes by, like security   agents at airports. And if there’s a problem with  one of your bags, wherever it is, cells from the   closest lymph node will sound the alarm.

Unfortunately, though, the immune system   is sometimes…a lil extra. It can flag things  that are perfectly harmless — like pollen,   certain foods, or even the body’s own cells.  And this is what causes allergies, asthma,   and some autoimmune diseases. It’s like,  immune system, I love you, but chill.  Overall, though, animals’ immune systems are   wildly impressive, in their ability to fight off  pathogens generally and to target specific ones.  From viruses to bacteria and everything  in-between, the world is full of organisms   trying to survive and reproduce… sometimes at the  expense of others.

But this is a battle that’s   been going on since the beginning of animals,  millions of years ago. And we are well-prepared,   hat or no hat. Next time, we’ll learn how   the body governs itself—how it communicates,  regulates, and coordinates all the processes   that keep us alive.

I’ll see you then. Peace!! This series was produced in collaboration with   HHMI BioInteractive.

If you’re an educator,  visit BioInteractive.org/Crashcourse for   classroom resources and professional development  related to the topics covered in this course.  Thanks for watching this episode of Crash  Course Biology which was filmed at our studio   in Indianapolis, Indiana, and was made with  the help of all these nice people. If you   want to help keep Crash Course free for everyone,  forever, you can join our community on Patreon.