Free Radicals - Michael Brooks [58]
In the early 1980s, Pritchard, who works at the University of Nottingham, was carrying out biology fieldwork in East Asia. He was trying to find evidence to back up anecdotal reports that people infected with hookworm, Necator americanus, were less susceptible to allergies. Allergies are ‘over-reactions’ of the immune system, and Pritchard wondered whether the worms were somehow able to turn off, or at least turn down, the immune response. If that were indeed the case, chronic allergy sufferers might get much-needed relief from the humble hookworm.
It was much later, in 2004, that Pritchard decided that the best way to find out would be to let the creatures feed on him. He put a dozen or so of the pin-sized worm larvae on a sticking plaster, and stuck it to his forearm. The larvae secreted an enzyme which broke down the molecular structure of his skin. Then they burrowed in. The burrowing produces an ‘intense itching’, Pritchard says. Once in, though, only the imagination is disturbed by their progress. The larvae were carried through his bloodstream. When they reached his lungs, they broke through the microscopically thin walls of the capillaries and colonised the alveolar spaces. From there they climbed upwards, into his trachea, until they reached the pharynx. Unconsciously, Pritchard swallowed the larvae. Safely lodged in his small intestine, the larvae grew into adults.
You are probably squirming at the thought of this parasitic invasion. Evolution has equipped us to take every measure – physical, chemical and emotional – to avoid such things. We have a natural reaction of disgust at the sight or smell of the human faeces that can harbour these parasites. Seeing (or imagining) the parasites themselves gives us an instinctive recoil that can save us from infection. An itch on the skin is similarly cautionary: that urge to touch it is a reaction that can swipe away danger. If none of these work, and the parasites do penetrate our defences, our immune system springs into action, attacking foreign invaders, causing us to cough to expel the alien presence in our throat, or to vomit if it is in our stomach.
But these parasites have evolved too. Once inside the human body, they turn off its alarm system – or at least they turn down its volume. By some as-yet unknown chemical mechanism, hookworm lavae suppress the immune response. And that means they get to stay.
That is why, with only a dozen or so worms inside him, Pritchard felt fine. His body did nothing to expel the adult worms as they passed beyond his stomach to his duodenum. There they attached themselves to the blood-rich wall, and siphoned off a small amount of his blood. Eventually, the males inseminated the females, larvae were produced, and Pritchard pooped them out.
Pritchard told his wife what he was up to. She was nervous about it, but that was then. These days, after dozens of cycles, she knows that a de-worming pill deals with her husband’s parasites. When he needs more, he can go to his colleague, Dr Alan Brown, whose body is home to a healthy population of hookworms, gathered – or rather, ingested – during the pair’s regular trips to Papua New Guinea (this, Pritchard explains, sidesteps the problem of customs officials wanting to know what is in their sample jars). As Pritchard put it in an unlikely sentence in The Biochemist, ‘His faecal cultures