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  • A mosquito lands on your arm,

  • injects its chemicals into your skin, and begins to feed.

  • You wouldn't even know it was there, if not for the red lump that appears,

  • accompanied by a telltale itch.

  • It's a nuisance, but that bump is an important signal

  • that you're protected by your immune system,

  • your body's major safeguard against infection, illness, and disease.

  • This system is a vast network of cells, tissues, and organs

  • that coordinate your body's defenses against any threats to your health.

  • Without it, you'd be exposed to billions of bacteria, viruses, and toxins

  • that could make something as minor as a paper cut or a seasonal cold fatal.

  • The immune system relies on millions of defensive white blood cells,

  • also known as leukocytes,

  • that originate in our bone marrow.

  • These cells migrate into the bloodstream and the lymphatic system,

  • a network of vessels which helps clear bodily toxins and waste.

  • Our bodies are teeming with leukocytes:

  • there are between 4,000 and 11,000 in every microliter of blood.

  • As they move around, leukocytes work like security personnel,

  • constantly screening the blood, tissues, and organs for suspicious signs.

  • This system mainly relies on cues called antigens.

  • These molecular traces on the surface of pathogens and other foreign substances

  • betray the presence of invaders.

  • As soon as the leukocytes detect them,

  • it takes only minutes for the body's protective immune response to kick in.

  • Threats to our bodies are hugely variable,

  • so the immune response has to be equally adaptable.

  • That means relying on many different types of leukocytes

  • to tackle threats in different ways.

  • Despite this diversity, we classify leukocytes in two main cellular groups,

  • which coordinate a two-pronged attack.

  • First, phagocytes trigger the immune response

  • by sending macrophages and dendritic cells into the blood.

  • As these circulate, they destroy any foreign cells they encounter,

  • simply by consuming them.

  • That allows phagocytes to identify

  • the antigen on the invaders they just ingested

  • and transmit this information to the second major cell group

  • orchestrating the defense,

  • the lymphocytes.

  • A group of lymphocyte cells called T-cells go in search of infected body cells

  • and swiftly kill them off.

  • Meanwhile, B-cells and helper T-cells

  • use the information gathered from the unique antigens

  • to start producing special proteins called antibodies.

  • This is the pièce desistance:

  • Each antigen has a unique, matching antibody that can latch onto it

  • like a lock and key,

  • and destroy the invading cells.

  • B-cells can produce millions of these,

  • which then cycle through the body and attack the invaders

  • until the worst of the threat is neutralized.

  • While all of this is going on, familiar symptoms,

  • like high temperatures and swelling,

  • are actually processes designed to aid the immune response.

  • A warmer body makes it harder for bacteria and viruses to reproduce and spread

  • because they're temperature-sensitive.

  • And when body cells are damaged,

  • they release chemicals that make fluid leak into the surrounding tissues,

  • causing swelling.

  • That also attracts phagocytes,

  • which consume the invaders and the damaged cells.

  • Usually, an immune response will eradicate a threat within a few days.

  • It won't always stop you from getting ill, but that's not its purpose.

  • Its actual job is to stop a threat

  • from escalating to dangerous levels inside your body.

  • And through constant surveillance over time,

  • the immune system provides another benefit:

  • it helps us develop long-term immunity.

  • When B- and T-cells identify antigens,

  • they can use that information to recognize invaders in the future.

  • So, when a threat revisits, the cells can swiftly deploy the right antibodies

  • to tackle it before it affects any more cells.

  • That's how you can develop immunity to certain diseases, like chickenpox.

  • It doesn't always work so well.

  • Some people have autoimmune diseases,

  • which trick the immune system

  • into attacking the body's own perfectly healthy cells.

  • No one knows exactly what causes them,

  • but these disorders sabotage the immune system to varying degrees,

  • and underlie problems like arthritis,

  • Type 1 diabetes,

  • and multiple sclerosis.

  • For most individuals, however,

  • a healthy immune system will successfully fight off an estimated 300 colds

  • and innumerable other potential infections over the course of a lifetime.

  • Without it, those threats would escalate into something far more dangerous.

  • So the next time you catch a cold or scratch a mosquito bite,

  • think of the immune system.

  • We owe it our lives.

A mosquito lands on your arm,

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B1 INT US TED-Ed immune immune system system immune response body

【TED-Ed】How does your immune system work? - Emma Bryce

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    IS LIU posted on 2018/03/05
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