An immune system of enormous complexity is present in all vertebrate animals. When we place a population of lymphocytes from such an animal in appropriate tissue culture fluid, and when we add an antigen, the lymphocytes will produce specific antibody molecules, in the absense of any nerve cells. I find it astonishing that the immune system embodies a degree of complexity which suggests some more or less superficial though striking analogies with human language, and that this cognitive system has evolved and functions without assistance of the brain.

The body has a very ingenious and usually effective system of natural defence against parasites, called the immune system. The immune system is so complicated that it would take a whole book to explain it. Briefly, when it senses a dangerous parasite the body is mobilized to produce special cells, which are carried by the blood into battle like a kind of army, tailor-made to attack the particular parasites concerned. Usually the immune system wins, and the person recovers. After that, the immune system 'remembers' the molecular equipment that it developed for that particular battle, and any subsequent infection by the same kind of parasite is beaten off so quickly that we don't notice it. That is why, once you have had a disease like measles or mumps or chickenpox, you're unlikely to get it again. People used to think it was a good idea if children caught mumps, say because the immune system's 'memory' would protect them against getting it as an adult - and mumps is even more unpleasant for adults (especially men, because it attacks the testicles) than it is for children. Vaccination is the ingenious technique of doing something similar on purpose. Instead of giving you the disease itself, the doctor gives you a weaker version of it, or possibly an injection of dead germs, to stimulate the immune system without actually giving you the disease. The weaker version is much less nasty than the real thing: indeed, you often don't notice any effect at all. But the immune system 'remembers' the dead germs, or the infection with the mild version of the disease, and so is forearmed to fight the real thing if it should ever come along.

The immune system has a difficult task 'deciding' what is 'foreign and therefore to be fought (a 'suspected' parasite), and what it should accept as part of the body itself. This can be particularly tricky, for example, when a woman is pregnant. The baby inside her is 'foreign' (babies are not genetically identical to their mothers because half their genes come from the father). But it is important for the immune system not to fight against the baby. This was one of the difficult problems that had to be solved when pregnancy evolved in the ancestors of mammals. It was solved -- after all, plenty of babies do manage to survive in the womb long enough to be born. But there are also plenty of miscarriages, which perhaps suggests that evolution had a hard time solving it and that the solution isn't quite complete. Even today, many babies survive only because doctors are on hand - for example, to change their blood completely as soon as they are born, in some extreme cases of immune-system overreaction.

Another way in which the immune system can get it wrong is to fight too hard against a supposed 'attacker'. That is what allergies are: the immune system needlessly, wastefully and even damagingly fighting harmless things. For example, pollen in the air is normally harmless, but the immune system of some people overreacts to it and that's when you get the allergic reaction called 'hay fever': you sneeze and your eyes water, and it is very unpleasant. Some people are allergic to cats, or to dogs: their immune systems are overreacting to harmless molecules in or on the hair of these animals. Allergies can sometimes be very dangerous. A few people are so allergic to peanuts that eating a single one can kill them.

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It is not surprising that the immune system sometimes overreacts, because there's a fine line to be trodden between failing to attack when you should and attacking when you shouldn't. It's the same problem we met over the antelope trying to decide whether to run away from the rustle in the long grass. Is it a leopard? Or is it a harmless puff" of wind stirring the grass? Is this a dangerous bacterium, or is it a harmless pollen grain? I can't help wondering whether people with a hyperactive immune system, who pay the penalty of allergies or even auto-immune diseases, might be less likely to suffer from certain kinds of viruses and other parasites.

Such 'balance' problems are all too common. It is possible to be too 'risk averse' - too jumpy. treating every rustle in the grass as danger, or unleashing a massive immune response to a harmless peanut or to the body's own tissues. And it is possible to be too gung-ho, failing to respond to danger when it is very real, or failing to mount an immune response when there really is a dangerous parasite. Treading the line is difficult, and there are penalties for straying off it in either direction.

The immune system consists of white blood cells that come in about 10 million different types. Each type has a protein lock on it called an "antibody," which corresponds to a key carried by a bacterium called an "antigen." If a key enters that lock, the white cell starts multiplying ferociously in order to produce an army of white cells to gobble up the key-carrying invader, be it a flu virus, a tuberculosis bacterium, or even the cells of a transplanted heart. But the body has a problem. It cannot keep armies of each antibody-lock ready to immobilize all types of keys because there is simply no room for millions of different types, each represented by millions of individual cells. So it keeps only a few copies of each white cell. As soon as one type of white cell meets the antigen that fits its locks, it begins multiplying. Hence the delay between the onset of flu and the immune response that cures it.

Each lock is generated by a sort of random assembly device that tries to maintain as broad a library of kinds of lock as it can, even if some of the keys that fit them have not yet been found in parasites. This is because the parasites are continually changing their keys to try to find ones that fit the host's changing locks. The immune system is therefore prepared. But this randomness means that the host is bound to produce white cells that are designed to attack its own cells among the many types it invents. To get around this, the host's own cells are equipped with a password, which is known as a major histocompatibility antigen. This stops the attack.

To win, then, the parasite must do one of the following: infect somebody else by the time the immune response hits (as flu does), conceal itself inside host cells (as the AIDS virus does), change its own keys frequently (as malaria does), or try to imitate whatever password the host's own cells carry that enable them to escape attention. Bilharzia parasites, for example, grab password molecules from host cells and stick them all over their bodies to camouflage themselves from passing white cells. Trypanosomes, which cause sleeping sickness, keep changing their keys by switching on one gene after another. The AIDS virus is craftiest of all. According to one theory it seems to keep mutating so that each generation has different keys. s. Time after time the host has locks that fit the keys and the virus gets suppressed. But eventually, after perhaps ten years, the virus's random mutation hits upon a key that the host does not have a lock for. At that point the virus has won. It has found the gap in the repertoire of the immune system's locks and runs riot. In essence, according to this theory the AIDS virus evolves until it finds a chink in the body's immune armor.