How Do Outbreaks Start? Pathogens and Immunology: Crash Course Outbreak Science #2
Summary
TLDRThis 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
🛡️ 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.
🌐 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.
🌟 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.
Mindmap
Keywords
💡Pathogens
💡Immune System
💡Viruses
💡Bacteria
💡Adaptive Immune System
💡Innate Immune System
💡Physical Barriers
💡Vaccines
💡Prions
💡Cytokines
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.
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