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By Root 459 0

for example, there are twenty-seven versions of five different genes that confer resistance to a rust fungus: twenty-seven versions of five locks. Each lock is fitted by several versions of one key gene in the rust. The virulence of the rust fungus attack is determined by how well its five keys fit the flax’s five locks. It is not quite like real keys and locks because there are partial fits: the rust does not have to open every lock before it can infect the flax. But the more locks it opens, the more virulent its effects.40

The Similarity between Sex and Vaccination

At this point the alert know-alls among my readers will be seething with impatience at my neglect of the immune system. The normal way to fight a disease, they point out, is not to have sex but to produce antibodies, by vaccination or whatever. The immune system is a fairly recent invention in evolutionary terms: it started in the reptiles perhaps 300 million years ago. Frogs, fish, insects, lobsters, snails and water fleas do not have immune systems. Even so, there is now an ingenious theory that marries the immune system with sex in an overarching Red Queen hypothesis. Hans Bremermann, of the University of California at Berkeley, is its author and he makes a fascinating case for the interdependence of the two. The immune system, he points out, would not work without sex.41

The immune system consists of white blood cells which come in about ten 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 so as 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 room for millions of cells of one type, or one cell each of millions of types, but not for millions of cells of millions of types. 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 you.

Each lock is generated by a sort of random assembly device, which tries to maintain as broad a library of kinds of lock as it can, even if some of the keys that fit them have never 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, known as a major histocompatibility antigen. This stops the attack. (Please excuse the mixed metaphor – keys and locks and passwords – it does not get any more mixed.)

To win, then, the parasite must either hope to infect somebody else by the time the immune response hits (as influenza 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. 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 just does not have a lock for. At that point, the virus has won. It has found the gap in the repertoire