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.
开始时你的喉咙有一点痒,接着你开始咳嗽, 肌肉变得疼痛, 情绪莫名的急躁, 变得没有胃口, 这些症状表明你得流感了。 我们理所当然的认为造成这一系列症状 的原因是身体受到了感染 但事实真的是这样吗? 到底是什么让我们感到身体不适? 如果是我们的身体在攻击它自己呢? 当流感病毒这样的病原体进入体内感染并杀死细胞时 你才会感到生病 但这种并不受欢迎的侵入却有另一种作用 它会使你身体的免疫系统注意于当下的状况 只要发生感染,免疫系统便立刻作出反应 巨噬细胞作为第一线的攻击者 将会搜索并消灭病毒与受感染的细胞 然后巨噬细胞会释放一种叫作细胞因子的蛋白质分子 它会从免疫系统中征集并组织更多的 细胞来杀死病毒 如果这一阶段的协调作用足够强的话 它会在你丝毫不知情的状态下清除感染 但这只是你身体为一些实际行动所作出的准备 在某些情况下,病毒进一步蔓延 甚至进入血液和重要器官 为了避免这种可能发生的危险 你的免疫系统必须发起更强的进攻 协调与大脑的活动 这就是那些引起不适的症状来源。 体温开始升高, 身体疼痛, 和昏昏欲睡。 所以我们为何会经历这些? 当免疫系统受到严重攻击时, 它分泌更多细胞因子 来触发两种反应: 首先,贯穿整个身体的脑神经,迷走神经 快速将信息传递到脑干 路经一个控制疼痛过程的重要区域 其次,细胞因子穿过身体到达下丘脑 它是大脑中控制温度调节, 口渴, 饥饿, 在其他事情中还有 睡意。 当下丘脑接受到这种信息 它会产生另一种分子 叫做前列腺素E2,在战争中发挥了重要的作用 下丘脑发出信号来控制你的肌肉收缩 并导致体温上升 它还会让你昏昏欲睡 感受不到饥饿和口渴 但是这些不舒服的症状有什么意义呢? 我们目前还不清楚 但是有些理论认为这有助于恢复 温度升高可以降低病毒活性 帮助你的免疫系统摧毁病原体 睡眠可以让你的身体积蓄更多能量来对抗感染。 当你停止进食,你的肝脏可以吸收血液中大量的 细菌赖以生存的铁 有效的消灭细菌 感觉不到口渴会使你轻微脱水 减少通过打喷嚏, 咳嗽, 呕吐, 和腹泻传播的病菌。 不过值得注意的是,如果你没有喝足够的水 脱水会变得十分危险。 你会对身体的疼痛更加敏感, 特别要注意受感染的伤口可能会恶化 甚至影响你的整体状态。 除了身体上表现出的症状 疾病也会让你感到烦躁, 伤心, 和困惑。 这是因为细胞因子和前列腺素 可以到达你大脑中更高的结构 破坏神经递质活性, 如谷氨酸, 内啡肽, 血清素, 和多巴胺。 这种行为会影响控制情绪的大脑边缘系统 和进行推理的大脑皮层 所以它是人体自身的免疫反应 导致你每次生病时的不适反应 可惜,免疫系统并不是总能完美运行。 最值得注意的是,全球数以百万计的人患有自身免疫性疾病 因为他们的免疫系统将正常的身体信号视为威胁 所以对自身进行攻击。 但是对于大多数人, 免疫系统历经数百万年的进化 不会对自身产生排斥反应。 我们自身疾病所引发的症状令人不适 但总体来说,他们是一种古老的过程 并将在未来的世纪中继续保护我们的身体不受外界影响。