Why does your immune system go to war against harmless pollen?

In an allergy, the immune system makes a mistake: it treats a harmless substance, such as pollen, as if it were dangerous. The trigger is an antibody called IgE. After a first encounter, the body makes IgE against the allergen, and these antibodies settle onto receptors on mast cells, cells packed with chemical granules. The next time the allergen arrives, it latches onto that IgE, and the mast cells release histamine and other mediators. That release is what produces sneezing, itching, swelling and the other familiar symptoms.

The most dramatic form of this reaction was discovered by surprise. In 1901, French physiologist Charles Richet and his colleague Paul Portier were studying a toxin from sea creatures such as the Portuguese man o' war. They expected a second dose to protect a dog, the way a vaccine would. Instead, the dog reacted violently. They called the surprising reaction anaphylaxis, and Richet won the 1913 Nobel Prize for the discovery. The antibody actually responsible, IgE, was not identified until 1966 and 1967, independently, by two research teams.

Allergies depend on both genes and environment. One debated idea, the hygiene hypothesis, proposes that too little early exposure to microorganisms leaves the immune system poorly tuned toward tolerance, making it more likely to overreact to harmless substances later. Some evidence supports a link, but many scientists still reject it.