The Immune System
An army of trillions of cells defends you against invaders you never notice, learns from every battle and remembers for decades. Vaccines train it; sometimes it turns on you.
Right now, on your skin, in your gut, in the air you just breathed, there are microbes that would kill you if they could. Almost none of them get the chance. Standing between you and them is the immune system, a defence force of trillions of cells and an arsenal of molecules, distributed throughout your body, that recognises invaders, destroys them, cleans up the mess and remembers the encounter for the rest of your life.
The first line
Most invaders never get in. Skin is a tough, dry, acidic wall that microbes cannot cross unless it is broken. Tears, saliva and mucus contain enzymes that kill bacteria. Stomach acid sterilises food. The airways are lined with mucus that traps particles and tiny hairs that sweep them out. The gut is crowded with friendly bacteria that leave no room for hostile ones. These barriers deal with the overwhelming majority of threats without the rest of the system ever being called.
The rapid response
When something does get through a cut or a scratch, the fast, general defence arrives within minutes. Damaged cells release chemicals that make nearby blood vessels leak, flooding the area with fluid and white cells, which is why a wound goes red, hot and swollen: that is inflammation, and it is a sign the system is working. Neutrophils, the most numerous white cells, swarm to the site and swallow bacteria whole, dying in the process; pus is mostly dead neutrophils. Larger cells called macrophages eat debris and invaders and, crucially, carry fragments of them away to show to the specialists. Other cells kill any of your own cells that have been infected by viruses or turned cancerous.
This innate system recognises general features that many microbes share. It is quick but not precise, and against some invaders it is not enough.
The specialists
The adaptive immune system takes days to get going but is exquisitely specific. Its soldiers are lymphocytes, about two trillion of them, of two kinds. B cells make antibodies, Y-shaped proteins whose tips are shaped to lock onto one particular target, a specific protein on the surface of a specific virus, say. Your body produces B cells with billions of different antibody shapes, almost at random, so that whatever invader turns up, a few of them will happen to fit. When one does, it multiplies furiously, and its clones pump out antibodies that coat the invader, mark it for destruction and stop it entering cells.
T cells are the commanders and assassins. Some coordinate the response, telling B cells to multiply and macrophages to attack harder. Others patrol the body checking every cell they meet for signs of infection, and kill the ones that are harbouring viruses before new viruses can be made.
Memory
After the battle, most of the clones die off, but some remain as memory cells, circulating for years or decades. If the same invader returns, they recognise it instantly and respond within hours instead of days, usually before you feel any symptoms. This is immunity, and it is why you catch chickenpox or measles only once.
Vaccines use this memory deliberately. A vaccine contains a harmless version or piece of a microbe: killed, weakened, or just a single protein, or instructions for your cells to make that protein briefly. The immune system responds as if to a real infection, builds memory, and is ready when the real thing arrives. Vaccination eradicated smallpox, a disease that killed hundreds of millions, and has driven polio, measles and tetanus to a fraction of their former toll. It is estimated to save several million lives a year.
When it goes wrong
The system must tell self from non-self, and it is remarkably good at it; T cells that would attack the body's own tissues are weeded out before they mature. Sometimes the weeding fails and the result is autoimmune disease: type 1 diabetes, where the insulin-making cells are destroyed; multiple sclerosis, where the insulation of nerves is attacked; rheumatoid arthritis, where the joints are. Allergies are the opposite error, a violent response to something harmless such as pollen, dust or peanut protein; in the extreme, anaphylaxis, the whole-body reaction can be fatal within minutes.
The system can also be overwhelmed or disabled. HIV destroys the very T cells that coordinate the defence, leaving the body open to infections it would normally shrug off. Chemotherapy and some drugs suppress it. And with age it slows and weakens, which is why the old are more vulnerable to infections and respond less strongly to vaccines.
Why it matters
You are alive because of a defence you never see. Every day it destroys invaders, clears out dead and damaged cells and probably eliminates early cancers before they can grow. Understanding it has given us vaccines, transplants, treatments for allergy and autoimmunity, and in the past decade a new generation of cancer therapies that work by unleashing the immune system on tumours. The army inside you is still the most sophisticated medicine there is.
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Test yourself
4 questions · answers explained at the end
1.What is the body's first line of defence?
2.What do antibodies do?
3.Why do you usually catch measles only once?
4.What is an allergy?