Why Do We Get Allergies?

The more you think about sickness and health, the trickier it gets to draw a clean line between them. We tend to think of ourselves as being prepared by nature for a good life. If we can just keep bacteria and viruses from killing us, and avoid walking into open elevator shafts, we’ll live a long, healthy life.

But we are actually the products of evolution, and evolution can’t give us perfect health. It has endowed us with powerful immune systems, thank you very much. And it has endowed us with quick reflexes that can, in some cases, keep us out of open elevator shafts. But evolution doesn’t automatically march to perfection. It stops short, leaving us with grave imperfections.

We have lots of defenses against cancer, for example, but they weaken as we get old. That’s a recipe for heartbreak in millions of families. But in the game of evolution, that’s a winning formula. Natural selection strongly favors defenses against cancer that threaten our ability to survive to adulthood and have kids. But if we die of cancer at age sixty, our kids are well on their way, carrying out genes down to the next generation.

This evolutionary perspective could change the way we think about our health in many ways. Take allergies. They affect millions of people, causing everything from hay fever to anaphylactic shock. One of the world’s leading immunologists, Ruslan Medzhitov, is convinced that allergies are actually adaptations we use to defend ourselves from noxious chemicals. As awful as allergies can get, we wouldn’t want to live without them.

I’ve written a profile of Medzhitov. It appeared today originally in Mosaic, but it’s now propagating through the Internet. You can also find it on Ars Technica, Discover, Gizmodo, Digg, and elsewhere. Check it out at the outlet of your choice. And good luck this pollen season!

6 thoughts on “Why Do We Get Allergies?

  1. Actually, in evolution, the survival of older members of the community is of considerable value in those species which have significant culture (that is, conveyance of information from the older to the younger members of the community). Certainly we observe this in humans but I believe it has been observed in other primates as well, and maybe in yet other species. The value of the elderly could be the reason for the menopause in humans: it would be more conducive to the survival of a community to keep the older women alive for longer, because of the value of their memories, than to exhaust them with continued childbearing, causing them to die at a younger age.

  2. But you don’t need many older people, and they benefit the whole tribe, not just their descendants or relatives.

    Therefore the value of their memories will provide at best a weak seletion pressure.

    1. So, then, what explains the menopause (which few other mammals exhibit) and what explains the powerful emotional connection humans experience toward their elders, who are often kept alive at considerable communal cost? Another evidence is the strong urge among humans to tell stories and recount personal experiences.

      I think the selection pressure might be pretty strong, since communities with greater aggregate memory might do considerable better than those without.

  3. One oxidative stress theory of aging says that it’s not just that our defenses against cancer weaken as a result of aging, but that aging itself is the very weakening of those defenses.

    It’s estimated our cells repair 20,000 DNA mutations per day caused by reactive oxygen species. If the rate were 100% perfect, we might not age past maturity or get cancer. But even a tiny error rate at our peak of apparent health leads to a long-term build up in errors, as the defense and repair mechanisms are damaged and become less effective.

  4. “So, then, what explains the menopause (which few other mammals exhibit) and what explains the powerful emotional connection humans experience toward their elders, who are often kept alive at considerable communal cost?”

    1. Such powerful emotional connections may be necessary to ensure even a tiny fraction of elders surviving, in the harsh environments early humans evolved in, where predation was high.

    2. There does not have to be an adaptive explanation for those powerful emotional bonds at all. They can simply be the side-effect of selection for the powerful emotional bonds that keep young children attached to their parents. There would be little evolutionary pressure to REMOVE those bonds after they have served their purpose, so they linger into adulthood and old age. They no more need an adaptive explanation than nipples in men.

  5. “I think the selection pressure might be pretty strong,”

    But not strong enough to prevent evolved defences against cancer from weakening with advanced age.

    Perhaps strong enough to prevent those defences from weakening too much in your 40’s, when your children may be just starting to mature to breeding age and just starting to have babies of their own (assuming you have children in your 20’s), and most in need of help in raising them, but not strong enough to prevent the defences from weakening when you are in your 60’s, when your own children are in their 40’s and able to help your grandchildren who are just starting to have kids of their own.

    Perhaps stronger than the same selection pressures in other species like salmon or fruit flies, but never stronger than the selection pressures favoring cancer protection in children and young adults in prime child-bearing age.

    “Strong” is always a relative term when it comes to selection pressure. The weakest selection pressure can have massive effects if there are not many other competing pressures around. The strongest selection pressure can be canceled out and have no effect if there are many other equally strong pressures pulling in other directions at the same time.

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