人体如何抵抗病毒

How Does the Body Fight Off a Virus?

鼻病毒是引起普通感冒的众多病毒之一

病毒是一种具有传染性的微生物，需要有一个活的宿主来生存和繁殖。当一个病毒进入你的身体，它侵入并接管细胞，重定向它们产生更多的病毒。

我们的身体如何防御病毒?
当我们的身体受到病毒感染的攻击时，它们会启动一种复杂的防御机制，即“免疫反应”。我们的免疫系统被设计来识别构成我们身体的细胞，并击退任何外来入侵者，如病毒。

他们通过使用由不同类型的白细胞组成的防御细胞大军来做到这一点。我们每天在骨髓中制造大约10亿个这样的细胞。
被称为巨噬细胞(Macrophages)的白细胞一发现细菌就会摧毁它们。然而，如果病毒感染开始肆虐，我们就会使用一种更强大的防御T淋巴细胞和B淋巴细胞的方法进行反击。

抗体(Antibodies)是B细胞制造的一种特殊蛋白质。它们与病毒结合阻止病毒复制，并标记病毒，使其他血细胞知道要摧毁它们。
T细胞有不同的作用。有些人充当警犬，在发现入侵病毒时发出警报; 另一些则直接杀死感染了病毒的细胞，或帮助B细胞产生抗体。
一旦病毒被清除，少数这种特殊的B细胞和T细胞就会持续存在，并对被破坏的病毒保持准确的记忆。

这意味着我们的免疫系统准备好防止来自同一病毒的另一次感染，而不会意外地攻击人体自身的细胞。这就是所谓的“后天免疫”。例如，在儿童时期感染一次腮腺炎会使你终生产生免疫力。

科学家们利用这种天生的能力来制造疫苗(Vaccine)，这种疫苗使用死亡或较弱的病毒菌株来启动免疫系统，并在不引起实际感染的情况下刺激长期免疫力。

针对麻疹等高传染性病毒的儿童免疫接种计划尤其重要，因为这些感染可能会导致严重甚至致命的并发症。

为什么病毒会回来?
首先，许多像那些引起流感的病毒在复制的过程中能够迅速变异。每一种新病毒的基因都有微小但往往有利的变化，这些变化在病毒从一个人传播到另一个人的过程中积累起来。
在这个过程中，病毒改变了它的外观，我们的免疫记忆细胞挣扎着识别它，让病毒再一次自由地感染我们。
这就是为什么你可以一直感染流感——新的突变菌株不断发展，而免疫系统无法检测到。

流感病毒还具有能够感染从人类到鸟类到猪的各种宿主的优势。这种菌株在动物群体中传播时变得非常不同。这使得它们的免疫系统无法识别，在它们跳回人类的罕见情况下，它们可以迅速传播。

与流感不同的是，感冒并不是由一种单一的病毒引起的，而是由数百种不同的病毒中的任何一种引起的。这些病毒不像流感病毒那样突变，所以每感冒一次，免疫系统就会对引起感冒的病毒产生免疫力。然而，人们仍然会得周期性感冒，仅仅是因为他们不断遇到以前没有的新病毒。

其他的病毒，比如水痘病毒，在被免疫系统消灭之前就变得不活跃了。它们在我们体内处于休眠状态，但在以后的生活中，由于身体和情绪上的压力，它们会重新活跃起来，导致一种被称为带状疱疹（Shingles)的疾病。在儿童时期患有水痘的人中，有十分之一在成年后会出现带状疱疹。

另一种本能——由引起唇疱疹的单纯疱疹病毒使用的一种本能——是隐藏在被称为神经元的神经细胞中的免疫系统。当病毒被激素或神经元上的压力激活时，它会保持宿主神经元细胞的完整，但会沿着神经传递到皮肤。在这里，它破坏皮肤细胞，在嘴唇上形成水泡，从而排出病毒。

科学家认为这是一种病毒的进化生存策略。它检测到身体的变化，这可能威胁到它的生存，并移动到皮肤引起表面的爆发，释放病毒粒子来寻找一个新的，健康的宿主。

我们能找到治疗感冒的方法吗?
目前正在研发一种抗病毒药物，可以杀死多种病毒，就像抗生素杀死许多不同种类的细菌一样。
目前，抗病毒药物专门针对特定疾病，如艾滋病毒和肝炎。但是来自麻省理工学院的一个小组已经成功地在人体组织和老鼠身上进行了一种能针对15种不同病毒的抗病毒药物实验。
它是由两种天然蛋白质“连接”而成的——一种检测到病毒进入，另一种充当自杀开关杀死被感染细胞。
这些药物还没有在人体上进行测试。目前，研究仍在继续。
　

HOW DOES THE BODY FIGHT OFF A VIRUS

The rhinovirus is one of the many viruses that can cause the common cold
Viruses are infectious micro-organisms that require a living host to survive and multiply. When one enters your body, it invades and takes over cells, redirecting them to produce more of the virus.

How do our bodies defend us from viruses?

When our bodies come under attack from a viral infection they launch a sophisticated defence known as 'the immune response'. Our immune system is designed to recognise the cells that make up our bodies and repel any foreign invaders such as viruses.

They do this by using a huge army of defender cells which consist of different types of white blood cell. We make around a billion of them every day in our bone marrow.

White blood cells called macrophages destroy germs as soon as they detect them. However, if a viral infection begins to take hold we fight back using a more powerful defence of white cells called T and B lymphocytes.

Antibodies are a special protein made by B cells. They bind to a virus to stop it from replicating, and also tag viruses so that other blood cells know to destroy them.

T cells have different roles to play. Some act as guard dogs that raise the alarm when they detect invading viruses; others kill virus-infected cells directly, or help B cells to produce antibodies.

Once the virus has been cleared, a small number of these specialist B and T cells persist and retain an accurate memory of the destroyed virus.

Once the virus has been cleared, a small number of these specialist B and T cells persist and retain an accurate memory of the destroyed virus.

This means our immune systems are primed to prevent another infection from the same virus, without attacking the body's own cells by accident. This is known as 'acquired immunity'. Having a single infection with mumps during childhood will give you lifelong resistance, for example.

Scientists have harnessed this natural ability in order to create vaccines, which use dead or weaker strains of viruses to prime the immune system and stimulate long-term resistance without causing the actual infection.

Childhood immunisation programmes against highly infectious viruses, such as measles, are particularly important as these infections can cause serious and even fatal complications first time around.

Why do viruses come back?

Firstly, many viruses like those that cause flu are skilled at rapidly mutating as they replicate. Each new virus has small, but often advantageous changes, in its genes and these accumulate as the virus passes from one person to another.

During this process, the virus alters its appearance and our immune memory cells struggle to recognise it, leaving the virus free to infect us once more.

This is why you can keep catching the flu - new mutated strains constantly develop, which the immune system can't detect.

The flu virus also has the advantage of being able to infect a variety of hosts, from humans to birds to swine. The strains become very different during their transmission within animal groups. This makes them unrecognisable to the immune system and, on the rare occasion when they jump back to humans, they can spread rapidly.

Unlike the flu, colds are not caused a single type of virus but any one of hundreds of different types of virus that can infect the upper airways. These viruses don't mutate like the flu viruses, so with each cold the immune system becomes resistant to the virus that caused it. However, people will still get recurrent colds simply because they keep coming across new viruses they haven't had before.

Other viruses, such as the chickenpox virus, become inactive before they are destroyed by the immune system. They remain dormant in our bodies but can be reactivated in later life by physical and emotional stress, resulting in a condition called shingles. One in 10 people who have chickenpox as children will have shingles as adults.

Another technique - one used by the herpes simplex virus which causes cold sores - is to hide from the immune system in nerve cells called neurones. When the virus is activated by hormones or stress on the neurones, it leaves its host neurone cells intact but travels down the nerve to the skin. Here it destroys skin cells and forms blisters on the lips which discharge the virus.

Scientists have suggested that this is an evolutionary survival tactic for the virus. It detects changes in the body which could threaten its existence, and moves to the skin causing a superficial eruption, liberating virus particles to find a new, healthy host.

Will we ever find a cure for the common cold?

Progress is being made in creating an antiviral drug that could kill multiple viruses, in the same way that antibiotics kill many different types of bacteria.

At present, antivirals are tailored to specific diseases such as HIV and hepatitis. But a team from the Massachusetts Institute of Technology have created one that has successfully treated 15 different viruses in lab tests on human tissue and mice.

It was created by 'wiring together' two natural proteins - one that detected virus entry, and another that acted as a suicide switch that kills the infected cell.

The drugs have yet to be tested on humans. For now, the research continues.

For symptoms and more information, visit NHS Choices

BBC Science - How does the body fight off a virus? http://www.bbc.co.uk/science/0/22028517