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.
你一開始是喉嚨癢 而後轉為咳嗽 肌肉開始疼痛 逐漸煩躁不安 並且喪失食慾 事實証明:你得了流行性感冒! 合理地推斷,這些多種難受症狀 是流經你身體的感染所導致 但真的是那樣嗎? 到底是什麼讓你覺得生病了? 如果是身體自己 不得不發起這猛烈的攻擊呢? 發病初起,像流感病毒類的病原體 侵入你的身體 感染並殺死細胞 但這令人討厭的入侵有另一效應: 它讓身體的免疫系統 警覺到這危險狀況 一旦它察覺到感染 身體會迅速啟動防衛 巨噬細胞會當攻擊先鋒隊 衝向感染區 搜尋並殺死病毒及受感染的細胞 然後,巨噬細胞釋放 稱為 細胞因子 (cytokines) 的蛋白質分子 它的任務是從免疫系統中 動員並組織更多的搜捕病毒的細胞 如果這協同作用力夠強 它甚至在你覺得生病前 就會殲滅這感染 但這只是身體 為真正的戰爭做準備而已 某些狀況,病毒會進一步擴散 甚至侵入血液及重要器官 為避免這偶發的危險後果 免疫系統必須發動更強的反擊 協同腦部一起展開行動 那就是引起不適症狀之所在 開始時,體溫劇升 肌肉酸痛 及昏昏欲睡 為什麼我們有這感受呢? 當免疫系統遭受嚴重攻擊 它會分泌更多細胞因子 以激發兩種反應 第一,迷走神經是從全身 匯聚至腦部 迅速地傳遞訊息到腦幹 它會通過處理疼痛的重要區域附近 第二,細胞因子從全身運行到下視丘 這部份的腦是負責控制體溫、 口渴、飢餓,及睡眠等等 當下視丘接收到訊息 它會製造另一種分子 稱為「前列腺素E2」 讓它進入備戰 下視丘發出信號指示肌肉收縮 因而導致體溫上升 它也使你昏昏欲睡 以及喪失食慾和口渴感 但所有這些不適症狀 有何意義呢? 這個…. 我們還不確定 但有些推測它們會幫助復原 體溫升高可阻礙細菌 以及協助免疫系統殺死病原體 睡眠讓身體集中更多精力 以對抗感染 當停止進食, 肝臟就能在血中吸收很多鐵 因為鐵是細菌生存所必需 如此可有效地使之匱乏 口渴感降低 使身體呈現輕度脫水 減少經由打噴嚏、 咳嗽、嘔吐或腹瀉的傳播 但值得注意的是如果飲水不足 可能變成危險的脫水 連身體疼痛也會使你變得更敏感 讓你注意到有個感染的傷口 可能加重 或造成你目前的狀況 除了身體症狀外 生病也會令你感到煩躁、 憂傷以及思想混亂 那是因為細胞因子和前列腺素 能到達腦部更高層的組織 干擾神經傳導物質的運作 例如:穀胺酸 (glutamate)、 內啡肽 (endorphin)、 血清素 (serotonin) 及多巴胺 (dopamine) 這會影響一些區域,如 監督情緒的邊緣系統(Limbic System) 及參與推理的大腦皮質 所以事實上是身體自己的免疫系統 導致每次感冒時的諸多不舒服 不幸的是,免疫系統並不總是運作完美 最值得一提的是全球有數百萬人 罹患「自體免疫性疾病」 也就是免疫系統 將正常身體信號視為威脅 因而攻擊自己的身體 但對大部份的人類而言 數百萬年來的演化 已將免疫系統精細調整 以便為人類效力,而非對抗 生病的症狀雖很惱人 但總體而論,它們象徵著一種久遠的過程 在未來的世紀,將繼續保衛我們的身體 對抗外在的世界