Many scientists have promoted the "hygiene hypothesis" (ie we are living in too clean an environment to prime our immune systems) as a reason for the growth in recent years of allergies and asthma.
But researchers in the UK and US think there may be another culprit missing ... worms!Scientists in Nottingham are investigating whether giving hook worms to asthma sufferers can improve their symptoms. In the US, pig worms are being introduced into patients with bowel and colon inflammation and Cambridge researchers have proven a link between a tropical worm and Type 1 diabetes in mice.
Worms have accompanied humans throughout the millions of years of our evolutionary history. It's only in recent times that we have found ways of eliminating them from our bodies.
"There is compelling evidence that something in our immune systems has changed since our ancestors, in fact has changed since our great-grandparents," Prof Danny Altman from London's Imperial College told the BBC.
Prof Graham Rook, from the University College London, said researchers now believed that the human immune system has become dependent of signals from certain organisms.
He said a recent study had demonstrated a similar sort of relationship.
Bacteria were introduced to a group of amoebae. The amoebae tried to kill the bacteria but could not. However after five years, neither organism could live without the other. The amoeba had deleted certain genes in their immune systems and the bacteria had done the same so they could co-exist peacefully. As a result, the amoeba no longer had a complete genome unless the bacteria were present.
"It now looks more and more likely that the development of our regulatory immune system depends on molecules that are encoded not in the genome of the human but in the genome of some other organism we lived with throughout history," Prof Rook said.He said there were now "good reasons" to think that a whole range of autoimmune disorders could be a result of our diminished exposure to our evolutionary partners.
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