How Do Outbreaks Start? Pathogens and Immunology: Crash Course Outbreak Science #2

Summary

This Crash Course Outbreak Science episode explores the human body's defenses against pathogens, including physical barriers like skin and mucus, and the immune system's two lines of defense: the innate response involving cells like macrophages and the adaptive response with B-cells and T-cells. It also touches on how vaccines leverage the adaptive immune system and the challenges pathogens pose, including disease transmission and immune system overreactions.

Takeaways

• 🛡️ The human body has evolved numerous defenses like skin, tear ducts, and hair in nostrils to protect against pathogens.
• 🦠 Pathogens are microscopic organisms like bacteria, viruses, protozoa, fungi, and prions that can cause diseases.
• 🦠 Viruses are unique pathogens that require host cells to reproduce and can cause a wide range of diseases.
• 🌐 Bacteria are single-celled organisms with circular genetic material; some are beneficial while others are pathogenic.
• 🐛 Protozoa are single-celled eukaryotes that can cause diseases like malaria when they enter the human body.
• 🍄 Fungi, including molds and yeasts, can be both beneficial and pathogenic, causing diseases like athlete's foot.
• 🐛 Parasitic worms are pathogenic animals that live inside humans, feeding off what they consume.
• 🧬 Prions are misfolded proteins that can cause other proteins to misfold, leading to diseases like Creutzfeldt-Jakob Disease.
• 🚪 The body has various entry points for pathogens, including obvious holes like mouth and nostrils, and less apparent ones like tear ducts.
• 🛡️ The immune system provides multiple layers of defense, including physical barriers like skin and mucus, and the innate immune system with cells like macrophages.
• 💉 Vaccines leverage the adaptive immune system's ability to remember pathogens, preparing the body to fight future infections more effectively.

Q & A

What is the primary function of the skin in defending against pathogens?

The skin acts as a physical barrier to prevent pathogens from entering the body. It is slightly acidic, which inhibits bacterial growth, and sweat contains enzymes that break down bacterial cell walls.

How do viruses reproduce if they are not considered fully 'living' organisms?

Viruses reproduce by infecting host cells and injecting their genetic material into the cell. They hijack the host cell’s resources to replicate themselves, which can disrupt the body's organs and cause sickness.

What are the main differences between bacteria and viruses?

Bacteria are single-celled organisms with genetic material free-floating inside them, while viruses lack cells and must infect host cells to reproduce. Some bacteria can be beneficial (like those in our gut), while viruses primarily cause illness.

What is the role of the immune system's innate defenses?

The innate immune system acts as a nonspecific barrier to detect and destroy invaders. It includes cells like monocytes, macrophages, and natural killer cells that patrol the body and eliminate pathogens.

What are prions, and why are they dangerous?

Prions are misfolded proteins that can cause other proteins to misfold as well. This leads to severe damage in the organ they affect, such as in Creutzfeldt-Jakob Disease (mad cow disease).

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

The adaptive immune system targets specific pathogens and has the ability to 'remember' them for quicker responses in future infections. B-cells and T-cells are key components of this system, producing antibodies and destroying infected cells.

What are the most common modes of pathogen transmission?

Pathogens can be transmitted through direct contact with bodily fluids, contaminated surfaces, airborne droplets, contaminated food and water, or by vectors like mosquitoes that inject pathogens into the bloodstream.

Why are memory B-cells and T-cells important in the immune response?

Memory B-cells and T-cells store information about pathogens they have encountered before, allowing for a faster and stronger immune response if the same pathogen invades again. This process is key in preventing reinfection.

What are some examples of pathogens that can be transmitted by vectors?

Malaria, caused by protozoa, is transmitted by mosquitoes, while other vectors like ticks and fleas can transmit diseases such as Lyme disease or plague.

How do vaccines support the immune system?

Vaccines help the immune system by exposing it to a harmless form or part of a pathogen, prompting the body to produce antibodies and memory cells. This prepares the immune system to defend against the actual pathogen in the future.

Outlines

00:00

🛡️ Pathogens and Our Body's Defenses

This paragraph introduces the concept of the human body as a fortress against pathogens, which are microscopic organisms that cause diseases. It explains that various body parts, such as skin, tear ducts, and nasal hairs, serve as defenses. Pathogens are diverse and can be categorized into viruses, bacteria, protozoa, fungi, and others like prions and parasitic worms. Each type has unique characteristics and methods of causing illness. The paragraph also sets the stage for the video series by introducing the host, Pardis Sabeti, and the theme of Crash Course Outbreak Science.

05:00

🌐 Pathogen Entry Points and Immune System Defenses

This section delves into how pathogens can enter the body through various 'holes' such as the mouth, nostrils, and even wounds. It discusses transmission methods like direct contact, contaminated surfaces, respiratory droplets, and ingestion. The paragraph then describes the body's immune system as a multi-layered defense mechanism, starting with physical barriers like skin and mucus, followed by the innate immune system with cells like monocytes, macrophages, and neutrophils. It also introduces the adaptive immune system, highlighting B-cells and T-cells, and the concept of immunological memory. The adaptive immune system's role in vaccine development is also mentioned, emphasizing its strategic approach to fighting pathogens.

10:01

🌟 Individual Immunity and the Broader Perspective

The final paragraph addresses the variability in how individuals respond to pathogens, noting that immune responses can differ greatly. It touches on the immune system's potential to overreact, leading to allergies and autoimmune disorders. The paragraph concludes by emphasizing the importance of understanding the immune system in managing disease outbreaks. It also acknowledges the support from various partners and encourages viewers to learn about Indigenous history and engage with local Indigenous communities. The video ends with a call to support Crash Course and a tease for future episodes that will explore the impact of population changes on disease dynamics.

Keywords

💡Pathogens

Pathogens are microscopic organisms that can cause disease, such as bacteria, viruses, protozoa, fungi, and prions. In the video, they are described as 'bugs' or 'germs' and are the primary agents responsible for making us sick. The script details the different types of pathogens and their effects on the human body, emphasizing the diversity of these organisms and their potential dangers.

💡Immune System

The immune system is the body's defense mechanism against pathogens, consisting of both physical barriers and specialized immune responses. The video describes how the immune system works in layers, starting with physical barriers like skin and mucus, followed by nonspecific responses like macrophages, and finally the adaptive immune system involving B-cells and T-cells. It is a central concept in understanding how the body fights infections.

💡Viruses

Viruses are fragments of genetic material wrapped in a protein coat, and they are unique because they cannot reproduce without a host cell. The script discusses how viruses take over host cells to replicate and cause diseases like the common cold, COVID-19, and smallpox. This explanation illustrates why viruses are often the focus in outbreak science due to their rapid spread and potential for causing epidemics.

💡Bacteria

Bacteria are single-celled organisms with genetic material that floats freely inside them, as opposed to being enclosed in a nucleus. Not all bacteria are harmful; some are beneficial, such as those in the human gut. The video contrasts pathogenic bacteria, which cause diseases like tuberculosis and cholera, with helpful bacteria that aid digestion and fermentation.

💡Adaptive Immune System

The adaptive immune system is a highly specialized part of the immune response that targets specific pathogens and has the ability to remember past infections. This allows the body to respond more effectively to future attacks. The video describes how B-cells and T-cells work together in this system, producing antibodies and destroying infected cells, providing a more targeted approach to pathogen defense.

💡Innate Immune System

The innate immune system is the body's nonspecific first line of defense against pathogens, involving cells like macrophages and neutrophils. These cells attack any invaders that get past physical barriers, acting as the guards of the body's fortress. The script explains how the innate immune system works quickly to identify and eliminate threats before they spread further.

💡Physical Barriers

Physical barriers are the body's first line of defense against pathogens, preventing them from entering the body. These include skin, mucus, eyelashes, and tears, all of which provide protection by either physically blocking pathogens or using antimicrobial substances to kill them. The video highlights how these barriers function like walls and moats of a fortress, safeguarding the body from infection.

💡Vaccines

Vaccines are medical tools that help the immune system recognize and fight specific pathogens without causing serious illness. The script mentions that vaccines use the adaptive immune system to prepare B-cells and T-cells for future encounters with a pathogen. This concept is crucial for understanding how vaccines prevent diseases by enabling faster and stronger immune responses upon re-exposure.

💡Prions

Prions are misfolded proteins that can cause other proteins in the body to become misshapen, leading to damage in organs, especially the brain. The video uses prions as an example of a less common, but highly dangerous pathogen that causes diseases like Creutzfeldt-Jakob Disease. Despite being rare, prions are significant because they challenge traditional definitions of infectious agents.

💡Cytokines

Cytokines are proteins released by immune cells to signal for help during an immune response. They play a crucial role in coordinating the body's defense mechanisms by attracting more immune cells to the site of infection. In the script, cytokines are described as distress signals released by macrophages and T-cells, facilitating communication between different parts of the immune system.

Highlights

The human body has evolved to defend against pathogens.

Pathogens are microscopic organisms that can cause disease.

Pathogens include bacteria, viruses, protozoa, fungi, and prions.

Viruses lack cells and must infect a host cell to reproduce.

Bacteria are single-celled organisms with genetic material in circular loops.

Protozoa are single-celled eukaryotes that can cause diseases like malaria.

Fungi can release spores that cause infections.

Prions are misfolded proteins that can cause other proteins to misfold.

The human body has physical barriers to prevent pathogens from entering.

The innate immune system provides a nonspecific defense against pathogens.

The adaptive immune system is highly specific and can adapt to fight pathogens.

B-cells produce antibodies that target specific pathogens.

T-cells recognize and destroy infected cells.

Immunological memory allows the immune system to respond more quickly to repeated infections.

Vaccines leverage the adaptive immune system to prepare the body to fight specific pathogens.

Pathogens can evade the immune system, leading to illness.

The immune system can overreact to non-threatening substances, causing allergies.

Autoimmune disorders occur when the immune system attacks the body's own cells.

Understanding the immune system is crucial for tackling diseases during outbreaks.

Transcripts

Your body is a fortress, crafted  through millions of years of evolution.  

And I don’t mean just your fists or your teeth.

The little things like your skin,  

tear ducts and even the hairs in your  nostrils are all designed to defend you.

That’s because they’re protecting  you from even tinier things:

pathogens, the microscopic  organisms that make us sick.

You might have heard of them in  vague terms like “bugs” or “germs,”

but the small world of pathogens  is actually incredibly diverse,

sometimes weird, and often, pretty dangerous.

And in this episode, we’re going to  get to know that world really well,

from the little creatures that live there to how  

our bodies protect us from the  ones that could make us sick.

I’m Pardis Sabeti, and this is  Crash Course Outbreak Science!

[Theme Music]

In our last episode, we saw how looking  

at infectious diseases from  a microbiology perspective

can help us understand and  tackle outbreaks better.

To a microbiologist, the roots  of disease are infectious agents,

the microbes and large molecules that are  transmitted between larger organisms, like humans.

To be more specific, it all starts with pathogens,

which is what we call the specific  infectious agents that can make us sick.

Pathogens tend to be microbes like  bacteria, viruses, protozoa and fungi.

There’s a huge variety of pathogens out there–

we’d need a whole other  series to describe them all!

but in general, they have a few key features  that help biologists tell them apart.

Let’s find out who’s who.

First up are viruses, which are made  up of fragments of genetic material,

wrapped in a kind of “coat” made of proteins.

Unlike most other pathogens,  they don’t have cells.

And depending on who you talk to, they  may not even qualify as living things!

That’s because they need to infect a  cell and use its resources to reproduce.

I’m team living thing myself.

They do this by latching onto a host cell,

injecting their own genetic material into it and  taking over the cell’s functions to multiply.

If they attack enough cells, viruses disrupt the  workings of our organs, causing us to get sick.

Smallpox, the common cold, flu, Ebola, Polio,  and COVID-19 are all caused by viruses.

Wow, that’s a lot of diseases!

That’s part of why we often focus  on viruses in outbreak science.

Our next microbe, bacteria, do have cells.

They’re single-celled organisms.

But while other kinds of cells keep  their genetic material inside a nucleus,

a bacteria’s genetic material is wrapped up in  circular loops that float freely inside them.

Not all bacteria are bad.

There’s friendly bacteria like the ones  in our stomachs that help us digest food,

and the ones we use to create  fermented foods like kimchi and yogurt.

But the pathogenic kind are much nastier.

Once inside the body, they can kill  your cells through direct attacks  

or by creating toxins that paralyze them.

Other kinds of bacteria multiply so  rapidly they damage entire organs!

That’s what the bacteria that cause  diseases like cholera and tuberculosis do.

Protozoa, the next microbes on our  list, are a little more like us.

They’re single-celled organisms,  yes, but they are eukaryotes,

which means they have a nucleus like our  cells do, and they’re undoubtedly alive.

When they get into our bodies, they can  harm us in ways similar to how bacteria can.

One of the most widespread  infectious diseases, malaria,  

is caused by protozoa carried by mosquitoes.

Then, there are fungi.

These are your molds, yeasts, and  mushrooms, and they’re also eukaryotes.

Some are made up of single  cells, some are multicellular,

some are harmless pizza  toppings, and some make us sick.

Fungi release tiny cells that can  reproduce on their own, called spores.

Certain kinds of pathogenic  spores travel easily in the air,

where they can stick to our skin or be inhaled.

During an infection, fungal cells multiply,

growing into places they shouldn’t  and feasting on the cells they infect!

Fungi are responsible for certain skin  diseases such as athlete’s foot and ringworm,

and other unpleasant things like oral thrush.

Finally, there are a few pathogen  oddballs, like parasitic worms.

Unlike the others, they’re animals…

Animals that live inside people  by feeding off what they eat.

They can even grow large enough  to be seen by the naked eye.

I won’t mince words here, it’s… pretty gross.

So let’s move on to the equally  weird and fascinating prions.

Prions are just proteins that have  ended up folded into the wrong shape.

It doesn’t sound like a bent-out-of-shape  protein would do much harm,

but they can be seriously dangerous!

If they come into contact with other,  correctly-folded proteins inside the body,

those proteins become misshapen too.

Those newly misshapen proteins bend  other proteins out of shape and so on,

damaging the organ they’re a part of.

That’s why we consider prion  diseases “infectious diseases.”

Prions can be inherited or  consumed in certain kinds of food.

One example is Creutzfeldt-Jakob Disease, or  mad cow disease, which occurs in the brain.

Okay, that sounds… terrifying.

But luckily, prions are super rare!

Microbiology is a pretty large field  and we’ve skimmed a lot of the details.

But it should give you an idea of the many  kinds of pathogens that might enter the body.

The question is… how do they do it?

On close inspection the  human body has lots of holes.

As I mentioned in our last episode,  

science demands clarity, so there’s  no shying away from the details here.

Some of the holes in the body are  obvious, like your mouth and nostrils.

Others aren’t as apparent, like your  tear ducts, ears, anus or genitals.

And although your skin is quite  a good barrier against pathogens,

tiny scratches, wounds or  bites can create holes too.

All of these holes are the routes  pathogens can take to get inside you.

For example, pathogens can be  transmitted by direct contact  

with an infected person’s skin or bodily fluids,

which is often the case for  sexually transmitted infections.

They can also be picked up from the  surfaces we touch with our hands,

and enter our bodies when we later  touch our eyes, mouth or nose.

Or an infected person might release droplets  containing pathogens when they talk,

cough or sneeze which then  get inhaled by someone else.

It could even be more straightforward!

Some pathogens find their  way into our food and water,

which we then unknowingly  put straight into our mouths.

Others, like malaria, are carried  by animals known as vectors.

Vectors are typically bloodsucking  arthropods, like mosquitos, ticks, and fleas,

and when one bites us to feed, they’re  basically creating another hole

through which they transmit pathogens  directly into our bloodstream.

It seems like the drawbridge is wide open for  invaders, as far as the human body is concerned,

hardly the most well protected fortress!

But your body has a whole host of  features to defend you from pathogens.

Together, these features form the immune system.

It all starts with physical barriers,  

which prevent pathogens from  entering in the first place.

Skin physically stops pathogens  from getting into our bodies.

What’s more, it’s slightly acidic, which  prevents bacteria from growing on it,

and our sweat contains enzymes that  break down bacterial cell walls.

Our eyes are similar.

Our eyelashes and eyelids physically prevent  airborne pathogens from reaching our eyes,

while our tears contain antimicrobial compounds  that kill anything our eyelashes miss.

Other potential entry holes into  the body, like our nostrils, lips,  

ears, genitals and anus are lined with mucus,

which physically traps pathogens, stopping  them from getting any further into you.

And though it might spread  disease if we’re already sick,

coughing and sneezing can eject unwanted  material from our airways that contain pathogens.

Finally, we can eject microbes out the other end.

Every time we use the bathroom,  

we’re also flushing out lots of  unwanted microbes from our systems.

These physical barriers are like the  walls, turrets and moats of the fortress,

providing a first line of defense.

But should any stubborn pathogens manage to break  through, the second line of defense kicks in:

the innate immune system.

This system has dedicated cells that attack any  trespassers, so we'll call it a nonspecific barrier.

Monocytes cruise along your bloodstream  looking for anything suspicious,

while macrophages and dendritic  cells keep an eye on your tissues.

If they find something, they  can digest the intruder.

And macrophages will eat anything dangerous  looking, even tattoo ink in your skin!

When a macrophage begins its fight,

it calls for help by releasing proteins  called cytokines as a distress signal.

At that point, tougher cells like  neutrophils and natural killer cells —

yes, that’s their real name —

will swoop in to help destroy tougher threats.

So the cells of the innate immune system  are like the guards of the fortress,

well trained to neutralize most enemies that make  it beyond the physical barriers of your body.

But occasionally, the body needs a more  specific approach in tackling a pathogen,

and calls for special forces.

That’s where the adaptive immune system comes in.

Unlike the innate immune system, the  adaptive immune system is highly specific.

Its cells target distinct  pathogens and continually, well,  

adapt to be stronger the next time.

Two important members of this specialized  team are the B-cells and T-cells.

B-cells are a type of white blood  cell that creates antibodies,

which are special, custom-made proteins  designed to stick onto one specific pathogen.

If an antibody binds the  pathogen it’s looking for,

the body triggers an immediate immune  response to rapidly destroy the threat.

That can look like blocking pathogens  from getting into our healthy cells,

or making pathogens clump together,

stopping them from infecting more cells and  making them easier for other immune cells to eat.

T-cells also look for specific pathogens,  but do it a little differently.

While B-cells and their antibodies  seek out pathogens directly,

T-cells recognize our own infected cells.

When they find one, they call in reinforcements:

Cytotoxic T-cells and Helper T-cells.

Cytotoxic T-cells are in charge  of destroying the infected cells,

while Helper T-cells coordinate  the rest of the response.

They help B-cells produce antibodies by nudging  them into action or releasing cytokines,

the protein distress signals  we talked about earlier.

Our adaptive immune system has a secret weapon  

that gives us an advantage against  repeated infections from the same pathogen.

It remembers pathogens it’s seen before so it  can recognize them more quickly the next time.

When T-cells or B-cells are exposed  to pieces of a digested pathogen  

they can specialize into memory cells.

This process is called immunological memory.

Memory T-cells are like historians,

documenting the invader’s attack and storing  that data in our bodies’ long term memory.

Memory B-cells, meanwhile, hang out in  the body after the first immune response,

ready to spot the pathogen and make  antibodies quickly if it shows up again.

The adaptive immune system is like an elite  guard of soldiers and military intelligence

that strategizes to defeat the  more serious threats to your body.

And it’s this system that we take  advantage of when we make vaccines.

They help our T and B cells recognize a particular  pathogen and prep to defend our bodies against it,

without making us seriously sick.

We’ll be talking about vaccines  in more detail in future episodes!

Unfortunately, even with all of these  remarkable layers of protection,

sometimes things can still go wrong.

Pathogens are often sneaky, and have multiple  ways of evading even our strongest defenses.

Sometimes we do get sick, or even get  sick multiple times from the same virus.

Our immunity also varies from person to person,

so what makes one person  too sick to get out of bed  

might look like it doesn’t  affect the next person at all!

Our immune system can even overreact to  something that isn’t actually a threat,

like a particle of pollen.

In that case, the body will  start up the immune response,

releasing the same cytokine  distress signals it normally would,

which can cause inflammation and swelling.

You might already know this  process by another name: allergies!

In the case of hayfever, it may just be annoying.

But a serious food allergy, for  example, could cause anaphylaxis,

when the throat swells up so  much that you can suffocate.

Similarly, an autoimmune  disorder, like Multiple Sclerosis,

is when a body is essentially allergic to  

itself and the immune system  attacks our own healthy cells.

On the whole though, the immune system does  

a remarkable job fending off the  many kinds of pathogens it faces.

Understanding these threats and  supporting the immune system is  

a crucial part of tackling  diseases during an outbreak.

Individual bodies are just  one part of the picture.

In our next episode, we’ll be zooming out  to look at how when groups of people change,

the way diseases affect them changes too.

We at Crash Course and our partners Operation Outbreak and the Sabeti Lab at the Broad Institute at MIT and Harvard

want to acknowledge the Indigenous people native to the land we live and work on,

and their traditional and ongoing relationship with this land.

We encourage you to learn about the history of the place you call home through resources like native-land.ca

and by engaging with your local Indigenous and Aboriginal nations

through the websites and resources they provide.

Thanks for watching this episode  of Crash Course Outbreak Science,

which was produced by Complexly in  partnership with Operation Outbreak

and the Sabeti Lab at the Broad  Institute of MIT and Harvard—

with generous support from the  Gordon and Betty Moore Foundation.

If you want to help keep Crash  Course free for everyone, forever,

you can join our community on Patreon.