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  • It starts with a tickle in your throat that becomes a cough.

  • Your muscles begin to ache,

  • you grow irritable,

  • and you lose your appetite.

  • It's official: you've got the flu.

  • It's logical to assume that this miserable medley of symptoms

  • is the result of the infection coursing through your body,

  • but is that really the case?

  • What's actually making you feel sick?

  • What if your body itself was driving this vicious onslaught?

  • You first get ill when a pathogen like the flu virus gets into your system,

  • infecting and killing your cells.

  • But this unwelcome intrusion has another effect:

  • it alerts your body's immune system to your plight.

  • As soon as it becomes aware of infection, your body leaps to your defense.

  • Cells called macrophages charge in as the first line of attack,

  • searching for and destroying the viruses and infected cells.

  • Afterwards, the macrophages release protein molecules called cytokines

  • whose job is to recruit

  • and organize more virus-busting cells from your immune system.

  • If this coordinated effort is strong enough,

  • it'll wipe out the infection before you even notice it.

  • But that's just your body setting the scene for some real action.

  • In some cases, viruses spread further,

  • even into the blood and vital organs.

  • To avoid this sometimes dangerous fate,

  • your immune system must launch a stronger attack,

  • coordinating its activity with the brain.

  • That's where those unpleasant symptoms come in,

  • starting with the surging temperature,

  • aches and pains,

  • and sleepiness.

  • So why do we experience this?

  • When the immune system is under serious attack,

  • it secretes more cytokines,

  • which trigger two responses.

  • First, the vagus nerve, which runs through the body into the brain,

  • quickly transmits the information to the brain stem,

  • passing near an important area of pain processing.

  • Second, cytokines travel through the body to the hypothalamus,

  • the part of the brain responsible for controlling temperature,

  • thirst,

  • hunger,

  • and sleep,

  • among other things.

  • When it receives this message,

  • the hypothalamus produces another molecule

  • called prostaglandin E2, which gears it up for war.

  • The hypothalamus sends signals that instruct your muscles to contract

  • and causes a rise in body temperature.

  • It also makes you sleepy,

  • and you lose your appetite and thirst.

  • But what's the point of all of these unpleasant symptoms?

  • Well, we're not yet sure,

  • but some theorize that they aid in recovery.

  • The rise in temperature can slow bacteria

  • and help your immune system destroy pathogens.

  • Sleep lets your body channel more energy towards fighting infection.

  • When you stop eating, your liver can take up much of the iron in your blood,

  • and since iron is essential for bacterial survival,

  • that effectively starves them.

  • Your reduced thirst makes you mildly dehydrated,

  • diminishing transmission through sneezes,

  • coughs,

  • vomit,

  • or diarrhea.

  • Though it's worth noting that if you don't drink enough water,

  • that dehydration can become dangerous.

  • Even the body's aches make you more sensitive,

  • drawing attention to infected cuts that might be worsening,

  • or even causing your condition.

  • In addition to physical symptoms,

  • sickness can also make you irritable,

  • sad,

  • and confused.

  • That's because cytokines and prostaglandin

  • can reach even higher structures in your brain,

  • disrupting the activity of neurotransmitters,

  • like glutamate,

  • endorphins,

  • serotonin,

  • and dopamine.

  • This affects areas like the limbic system, which oversees emotions,

  • and your cerebral cortex, which is involved in reasoning.

  • So it's actually the body's own immune response

  • that causes much of the discomfort you feel every time you get ill.

  • Unfortunately, it doesn't always work perfectly.

  • Most notably, millions of people worldwide suffer from autoimmune diseases,

  • in which the immune system treats normal bodily cues as threats,

  • so the body attacks itself.

  • But for the majority of the human race,

  • millions of years of evolution have fine-tuned the immune system

  • so that it works for, rather than against us.

  • The symptoms of our illnesses are annoying,

  • but collectively, they signify an ancient process

  • that will continue barricading our bodies against the outside world

  • for centuries to come.

It starts with a tickle in your throat that becomes a cough.

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B2 TED-Ed immune immune system body system hypothalamus

【TED-Ed】The surprising reason you feel awful when you're sick - Marco A. Sotomayor

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    黃于珍 posted on 2016/05/04
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