Yale researchers suggested Wednesday that allergies may be an outgrowth of the protective mechanism of the body to protect itself from noxious substances in the environment.
Humans and other mammals have two major types of immunity; intriguingly, neither birds nor reptiles share these immune defenses.
- Type 1 immunity involves the defence against viruses, bacteria and the like. It is an active defence in which cells from the immune system engulf and destroy the invaders.
- Type 2 immunity is traditionally held to be directed against larger parasites, such as helminth worms and biting spiders. It is more of a passive defence in which the body erects barriers against invasion.
According to Immunobiologist Ruslan Medzhitov of Yale University and his colleagues in their perspective paper published in the current issue of Nature, there are several problems with this view of isolating the two types of immunity. First and foremost, most allergens do not actually have any obvious relationship with parasitic worms. Second, the type 2 immunity, manifested as anaphylaxis, occurs extremely rapidly, even though there is no need for such a speedy response to the slowly multiplying parasites. And finally, allergic hypersensitivity can develop against a wide variety of allergens that have little in common in terms of their structure or origin: pollen, shellfish, peanuts, bee venom, latex, penicillin and nickel.
According to Medzhitov the type 2 immunity actually evolved against a broader variety of stimuli, including venoms, natural toxins and irritants. A runny nose and sneezing help flush such irritants out of the nose and lungs. Scratching promotes removal of toxins from the skin. Such reactions also serve as a signal to avoid places where stinging insects or environmental toxins are common.
Food allergy symptoms often include vomiting and diarrhea, which also expel unwanted substances. Anaphylactic shock, the most severe and deadly reaction, involves swelling of the mouth or throat and even breathing problems. Medzhitov explains this as an extreme form of the normal allergic reaction; it has gone out of control and may cause death in and of itself, the way that septic shock is an extreme form of natural inflammation.
There’s even been a hint in the scientific literature that people who are allergy prone are less likely to develop certain cancers, Medzhitov said. “You can imagine if there’s some toxic substance in the environment that can be carcinogenic you might be better protected,” he adds.
“We believe that allergic hypersensitivity evolved to survey the environment for the presence of noxious substances,” Medzhitov said. “After the first exposure, the immune system gains a memory, and subsequent exposure to even minute amounts will induce an anticipatory response that helps minimize potentially harmful effects....According to this view, hypersensitivity to allergens triggers avoidance of a sub-optimal environment.”
Dr. Robert Wood, Director of Allergy/Immunology at Johns Hopkins Children's Center, who was not involved in this study said, “This is a fascinating paper that sheds new light on why some people develop allergies and others do not. It also suggests reasons why the prevalence of allergic disease, including food allergy, asthma, and hay fever, have all increased over the past 20-30 years.” Wood added, “Hopefully the theories presented here could eventually be used to design prevention strategies for allergy, but at the present time they remain just theories that will not lead to any immediate approaches to treat or prevent allergy.”