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
- š”ļø The human body has evolved numerous defenses like skin, tear ducts, and hair in nostrils to protect against pathogens.
- š¦ Pathogens are microscopic organisms like bacteria, viruses, protozoa, fungi, and prions that can cause diseases.
- š¦ Viruses are unique pathogens that require host cells to reproduce and can cause a wide range of diseases.
- š Bacteria are single-celled organisms with circular genetic material; some are beneficial while others are pathogenic.
- š Protozoa are single-celled eukaryotes that can cause diseases like malaria when they enter the human body.
- š Fungi, including molds and yeasts, can be both beneficial and pathogenic, causing diseases like athlete's foot.
- š Parasitic worms are pathogenic animals that live inside humans, feeding off what they consume.
- š§¬ Prions are misfolded proteins that can cause other proteins to misfold, leading to diseases like Creutzfeldt-Jakob Disease.
- šŖ The body has various entry points for pathogens, including obvious holes like mouth and nostrils, and less apparent ones like tear ducts.
- š”ļø The immune system provides multiple layers of defense, including physical barriers like skin and mucus, and the innate immune system with cells like macrophages.
- š 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 cellās 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
š”ļø 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.
š 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.
š 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
š”Pathogens
š”Immune System
š”Viruses
š”Bacteria
š”Adaptive Immune System
š”Innate Immune System
š”Physical Barriers
š”Vaccines
š”Prions
š”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, craftedĀ through millions of years of evolution. Ā
And I donāt mean just your fists or your teeth.
The little things like your skin,Ā Ā
tear ducts and even the hairs in yourĀ nostrils are all designed to defend you.
Thatās because theyāre protectingĀ you from even tinier things:
pathogens, the microscopicĀ organisms that make us sick.
You might have heard of them inĀ vague terms like ābugsā or āgerms,ā
but the small world of pathogensĀ is actually incredibly diverse,
sometimes weird, and often, pretty dangerous.
And in this episode, weāre going toĀ get to know that world really well,
from the little creatures that live there to howĀ Ā
our bodies protect us from theĀ ones that could make us sick.
Iām Pardis Sabeti, and this isĀ Crash Course Outbreak Science!
[Theme Music]
In our last episode, we saw how lookingĀ Ā
at infectious diseases fromĀ a microbiology perspective
can help us understand andĀ tackle outbreaks better.
To a microbiologist, the rootsĀ of disease are infectious agents,
the microbes and large molecules that areĀ transmitted between larger organisms, like humans.
To be more specific, it all starts with pathogens,
which is what we call the specificĀ infectious agents that can make us sick.
Pathogens tend to be microbes likeĀ bacteria, viruses, protozoa and fungi.
Thereās a huge variety of pathogens out thereā
weād need a whole otherĀ series to describe them all!
but in general, they have a few key featuresĀ that help biologists tell them apart.
Letās find out whoās who.
First up are viruses, which are madeĀ up of fragments of genetic material,
wrapped in a kind of ācoatā made of proteins.
Unlike most other pathogens,Ā they donāt have cells.
And depending on who you talk to, theyĀ may not even qualify as living things!
Thatās because they need to infect aĀ cell and use its resources to reproduce.
Iām team living thing myself.
They do this by latching onto a host cell,
injecting their own genetic material into it andĀ taking over the cellās functions to multiply.
If they attack enough cells, viruses disrupt theĀ workings of our organs, causing us to get sick.
Smallpox, the common cold, flu, Ebola, Polio,Ā and COVID-19 are all caused by viruses.
Wow, thatās a lot of diseases!
Thatās part of why we often focusĀ on viruses in outbreak science.
Our next microbe, bacteria, do have cells.
Theyāre single-celled organisms.
But while other kinds of cells keepĀ their genetic material inside a nucleus,
a bacteriaās genetic material is wrapped up inĀ circular loops that float freely inside them.
Not all bacteria are bad.
Thereās friendly bacteria like the onesĀ in our stomachs that help us digest food,
and the ones we use to createĀ fermented foods like kimchi and yogurt.
But the pathogenic kind are much nastier.
Once inside the body, they can killĀ your cells through direct attacksĀ Ā
or by creating toxins that paralyze them.
Other kinds of bacteria multiply soĀ rapidly they damage entire organs!
Thatās what the bacteria that causeĀ diseases like cholera and tuberculosis do.
Protozoa, the next microbes on ourĀ list, are a little more like us.
Theyāre single-celled organisms,Ā yes, but they are eukaryotes,
which means they have a nucleus like ourĀ cells do, and theyāre undoubtedly alive.
When they get into our bodies, they canĀ harm us in ways similar to how bacteria can.
One of the most widespreadĀ infectious diseases, malaria,Ā Ā
is caused by protozoa carried by mosquitoes.
Then, there are fungi.
These are your molds, yeasts, andĀ mushrooms, and theyāre also eukaryotes.
Some are made up of singleĀ cells, some are multicellular,
some are harmless pizzaĀ toppings, and some make us sick.
Fungi release tiny cells that canĀ reproduce on their own, called spores.
Certain kinds of pathogenicĀ 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 shouldnātĀ and feasting on the cells they infect!
Fungi are responsible for certain skinĀ diseases such as athleteās foot and ringworm,
and other unpleasant things like oral thrush.
Finally, there are a few pathogenĀ oddballs, like parasitic worms.
Unlike the others, theyāre animalsā¦
Animals that live inside peopleĀ by feeding off what they eat.
They can even grow large enoughĀ to be seen by the naked eye.
I wonāt mince words here, itāsā¦ pretty gross.
So letās move on to the equallyĀ weird and fascinating prions.
Prions are just proteins that haveĀ ended up folded into the wrong shape.
It doesnāt sound like a bent-out-of-shapeĀ protein would do much harm,
but they can be seriously dangerous!
If they come into contact with other,Ā correctly-folded proteins inside the body,
those proteins become misshapen too.
Those newly misshapen proteins bendĀ other proteins out of shape and so on,
damaging the organ theyāre a part of.
Thatās why we consider prionĀ diseases āinfectious diseases.ā
Prions can be inherited orĀ consumed in certain kinds of food.
One example is Creutzfeldt-Jakob Disease, orĀ mad cow disease, which occurs in the brain.
Okay, that soundsā¦ terrifying.
But luckily, prions are super rare!
Microbiology is a pretty large fieldĀ and weāve skimmed a lot of the details.
But it should give you an idea of the manyĀ kinds of pathogens that might enter the body.
The question isā¦ how do they do it?
On close inspection theĀ human body has lots of holes.
As I mentioned in our last episode,Ā Ā
science demands clarity, so thereāsĀ no shying away from the details here.
Some of the holes in the body areĀ obvious, like your mouth and nostrils.
Others arenāt as apparent, like yourĀ tear ducts, ears, anus or genitals.
And although your skin is quiteĀ a good barrier against pathogens,
tiny scratches, wounds orĀ bites can create holes too.
All of these holes are the routesĀ pathogens can take to get inside you.
For example, pathogens can beĀ transmitted by direct contactĀ Ā
with an infected personās skin or bodily fluids,
which is often the case forĀ sexually transmitted infections.
They can also be picked up from theĀ surfaces we touch with our hands,
and enter our bodies when we laterĀ touch our eyes, mouth or nose.
Or an infected person might release dropletsĀ containing pathogens when they talk,
cough or sneeze which thenĀ get inhaled by someone else.
It could even be more straightforward!
Some pathogens find theirĀ way into our food and water,
which we then unknowinglyĀ put straight into our mouths.
Others, like malaria, are carriedĀ by animals known as vectors.
Vectors are typically bloodsuckingĀ arthropods, like mosquitos, ticks, and fleas,
and when one bites us to feed, theyāreĀ basically creating another hole
through which they transmit pathogensĀ directly into our bloodstream.
It seems like the drawbridge is wide open forĀ invaders, as far as the human body is concerned,
hardly the most well protected fortress!
But your body has a whole host ofĀ features to defend you from pathogens.
Together, these features form the immune system.
It all starts with physical barriers,Ā Ā
which prevent pathogens fromĀ entering in the first place.
Skin physically stops pathogensĀ from getting into our bodies.
Whatās more, itās slightly acidic, whichĀ prevents bacteria from growing on it,
and our sweat contains enzymes thatĀ break down bacterial cell walls.
Our eyes are similar.
Our eyelashes and eyelids physically preventĀ airborne pathogens from reaching our eyes,
while our tears contain antimicrobial compoundsĀ that kill anything our eyelashes miss.
Other potential entry holes intoĀ the body, like our nostrils, lips,Ā Ā
ears, genitals and anus are lined with mucus,
which physically traps pathogens, stoppingĀ them from getting any further into you.
And though it might spreadĀ disease if weāre already sick,
coughing and sneezing can eject unwantedĀ material from our airways that contain pathogens.
Finally, we can eject microbes out the other end.
Every time we use the bathroom,Ā Ā
weāre also flushing out lots ofĀ unwanted microbes from our systems.
These physical barriers are like theĀ walls, turrets and moats of the fortress,
providing a first line of defense.
But should any stubborn pathogens manage to breakĀ through, the second line of defense kicks in:
the innate immune system.
This system has dedicated cells that attack anyĀ trespassers, so we'll call it a nonspecific barrier.
Monocytes cruise along your bloodstreamĀ looking for anything suspicious,
while macrophages and dendriticĀ cells keep an eye on your tissues.
If they find something, theyĀ can digest the intruder.
And macrophages will eat anything dangerousĀ looking, even tattoo ink in your skin!
When a macrophage begins its fight,
it calls for help by releasing proteinsĀ called cytokines as a distress signal.
At that point, tougher cells likeĀ neutrophils and natural killer cells ā
yes, thatās their real name ā
will swoop in to help destroy tougher threats.
So the cells of the innate immune systemĀ are like the guards of the fortress,
well trained to neutralize most enemies that makeĀ it beyond the physical barriers of your body.
But occasionally, the body needs a moreĀ specific approach in tackling a pathogen,
and calls for special forces.
Thatās where the adaptive immune system comes in.
Unlike the innate immune system, theĀ adaptive immune system is highly specific.
Its cells target distinctĀ pathogens and continually, well,Ā Ā
adapt to be stronger the next time.
Two important members of this specializedĀ team are the B-cells and T-cells.
B-cells are a type of white bloodĀ cell that creates antibodies,
which are special, custom-made proteinsĀ designed to stick onto one specific pathogen.
If an antibody binds theĀ pathogen itās looking for,
the body triggers an immediate immuneĀ response to rapidly destroy the threat.
That can look like blocking pathogensĀ from getting into our healthy cells,
or making pathogens clump together,
stopping them from infecting more cells andĀ making them easier for other immune cells to eat.
T-cells also look for specific pathogens,Ā but do it a little differently.
While B-cells and their antibodiesĀ 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 chargeĀ of destroying the infected cells,
while Helper T-cells coordinateĀ the rest of the response.
They help B-cells produce antibodies by nudgingĀ them into action or releasing cytokines,
the protein distress signalsĀ we talked about earlier.
Our adaptive immune system has a secret weaponĀ Ā
that gives us an advantage againstĀ repeated infections from the same pathogen.
It remembers pathogens itās seen before so itĀ can recognize them more quickly the next time.
When T-cells or B-cells are exposedĀ to pieces of a digested pathogenĀ Ā
they can specialize into memory cells.
This process is called immunological memory.
Memory T-cells are like historians,
documenting the invaderās attack and storingĀ that data in our bodiesā long term memory.
Memory B-cells, meanwhile, hang out inĀ the body after the first immune response,
ready to spot the pathogen and makeĀ antibodies quickly if it shows up again.
The adaptive immune system is like an eliteĀ guard of soldiers and military intelligence
that strategizes to defeat theĀ more serious threats to your body.
And itās this system that we takeĀ advantage of when we make vaccines.
They help our T and B cells recognize a particularĀ pathogen and prep to defend our bodies against it,
without making us seriously sick.
Weāll be talking about vaccinesĀ in more detail in future episodes!
Unfortunately, even with all of theseĀ remarkable layers of protection,
sometimes things can still go wrong.
Pathogens are often sneaky, and have multipleĀ ways of evading even our strongest defenses.
Sometimes we do get sick, or even getĀ sick multiple times from the same virus.
Our immunity also varies from person to person,
so what makes one personĀ too sick to get out of bedĀ Ā
might look like it doesnātĀ affect the next person at all!
Our immune system can even overreact toĀ something that isnāt actually a threat,
like a particle of pollen.
In that case, the body willĀ start up the immune response,
releasing the same cytokineĀ distress signals it normally would,
which can cause inflammation and swelling.
You might already know thisĀ process by another name: allergies!
In the case of hayfever, it may just be annoying.
But a serious food allergy, forĀ example, could cause anaphylaxis,
when the throat swells up soĀ much that you can suffocate.
Similarly, an autoimmuneĀ disorder, like Multiple Sclerosis,
is when a body is essentially allergic toĀ Ā
itself and the immune systemĀ attacks our own healthy cells.
On the whole though, the immune system doesĀ Ā
a remarkable job fending off theĀ many kinds of pathogens it faces.
Understanding these threats andĀ supporting the immune system isĀ Ā
a crucial part of tacklingĀ diseases during an outbreak.
Individual bodies are justĀ one part of the picture.
In our next episode, weāll be zooming outĀ 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 episodeĀ of Crash Course Outbreak Science,
which was produced by Complexly inĀ partnership with Operation Outbreak
and the Sabeti Lab at the BroadĀ Institute of MIT and Harvardā
with generous support from theĀ Gordon and Betty Moore Foundation.
If you want to help keep CrashĀ Course free for everyone, forever,
you can join our community on Patreon.
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