The Immune System Reaction and Overreaction
It is a delicate balance, as when a mother is carrying a baby and her immune system must not be allowed to attack it, or when people get hay fever or allergies.
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.
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.