Monday, 28 July 2008


Author's note: Analogy is wonderful thing. Let's do some analogy about allergy. Oh man I'm sorry. Really the main point is to do with how context is important in evolution.

Current speculative thinking is that the immune systems of many individuals co-evolved in the presence of persistent parasitic infestation. The immune system therefore evolved to over-compensate along the anti-parasite axis. It needed to be able to deal with new parasitic infection on top of the un-clearable persistent infection. We call this parasite killing axis the Th2 response. We can imagine our immune system as a sort of see-saw, but with many sides. As one side, or axis of our immune response raises, it pushes the others down. Some infections require a broad, balanced response. Others need a strong, single axis attack. Our persistent parasites evolved, adapting to evade our Th2 response. Some would push our immune response along other axes, perhaps towards our anti-bacterial response. The upshot of this was that we required further Th2 over-compensation. This co-evolution has probably been ongoing since our pre-mammalian ancestors. This is evolution over a time on the order of hundreds of millions of years.

Suddenly, in the space of a mere 200 years, the western world eliminated normal everyday parasitic infection. From an evolutionary point of view, an advantageous trait had become redundant in the blink of an eye. Individuals who previously had an evolutionary edge suddenly had a disadvantage.
The asthamtic, the hayfever sufferer and the general sneezy snot bag has an immune system that resembles a race horse suddenly lacking its burden. The jockey, that nasty little parasite, has fallen off. The horse is gleefully running for the finish line, thinking he is about to win.

This stands as a wonderful example of the importance of context in evolutionary traits and in the emergence of new mutations. Allergy is today seen as some sort of genetic "defect". In African countries where the parasitic trypanosome and schistosome problems are finally starting to be reduced, allergy is starting to emerge. Perhaps this is mere coincidence, but if our thinking is correct, allergy and asthma will increase considerably there over the coming years. That is of course assuming that the western world gets off its arse to help do something about health in Africa.

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