The Thorn in the Starfish: How the Human Immune System Works
The great geneticist Theodosius Dobzhansky wrote that in nature nothing makes sense unless we bear in mind that natural selection reigns supreme. In Africa, one of the sources of poverty is a cattle disease caused by a parasite, the trypanosome. The disease is transmitted by the tsetse fly. When this fly stings a cow, the trypanosomes penetrate into her blood, where they are recognized as foreign invaders by some of her white blood cells. Alarmed by that signal, these particular white cells divide and multiply, and their descendants secrete antibodies into the blood that kill the parasites. Alas, not quite all of them. A few survive because genetic mutations have dressed them up in new coats unrecognized by the cow’s antibodies; these survivors now divide and multiply, and force the cow’s immune system to begin the fight all over again. The same battle repeats itself every few weeks.
The Dutch molecular biologist Piet Borst discovered the genetic mechanism that enables the trypanosomes to take up a multitude of different disguises. He found that their chromosomes contain a repertoire of genetic “cassettes,” each capable of directing the manufacture of a different protein coat; mutations can activate such cassettes in turn by inserting them into the same “cassette player.” None of these new coats can fool the cow’s defenses for long because early in life the genes that code for its antibodies have been shuffled in about a hundred million different ways, allowing the cow to make about a hundred million different antibodies, each secreted by a different population of white blood cells. This profligacy ensures that the cow can make antibodies not only against the trypanosomes in all their different guises, but also against all other conceivable infections.
The mutations that change the trypanosomes’ coats, and the shuffling of the genes that gives rise to millions of different antibodies, are chance events. Natural selection causes those of the cow’s white cells that by chance recognize the trypanosomes, and those of the trypanosomes that escape recognition at first, to divide and multiply. The Darwinian struggle between the white cells and the trypanosomes ensures the survival of the population of parasites and also that of their host, the cow, but she becomes emaciated and a poor producer of milk, to the detriment of the farmer.
White blood cells like those that respond to the trypanosomes are the soldiers that spring to an animal’s defense on infection. Robert S. Desowitz’s book introduces the layman to their different uniforms, weapons, and tactics, and to the systems of command that control them, including, surprisingly, our state of mind. The vicious cold that struck you just before your final exams may have penetrated your defenses because mental stress and exhaustion had already made your immune system crumble. Experiments on rats suggest that the immune response can even be suppressed by a conditioned reflex, as if, after a series of exams, the mere sight of another examination paper could suppress it.
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The Vaccine Business March 3, 1988