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WE NOW know why a whiff of pollen or a hint of peanut can cause such severe allergic reactions. The discovery should make it possible to design more effective drugs.

Most anti-allergy drugs on the market simply subdue the symptoms. Antihistamines, for example, can fight off red eyes and runny noses, but they can have side effects. Plus, the more you use, the more you need to get the same effect.

Most of the irritating effects are triggered by antibodies called IgEs. These are present in everyone鈥檚 immune systems, though a particularly allergy-prone person can have thousands of times more than someone else. The Y-shaped IgE antibodies attach themselves to hosts called mast cells, and when their upper arms pick up an allergen they tell the mast cell to pump out irritants such as histamines.

Most antibodies cling onto their hosts for only a few minutes, but IgE antibodies can hang on to mast cells for weeks. Now biologist Brian Sutton at King鈥檚 College London and his team have discovered why. They found that the top of the Y-shaped IgE antibody is bent back on itself, and that when it attaches to a mast cell it unbends by about 90 degrees, which somehow locks it in place.

If you could design a drug that keeps the antibodies in their bent shape, Sutton says, you could stop them binding to mast cells or even pop them off, stopping allergic symptoms. And past studies have shown that when IgE antibodies stop pestering mast cells, the mast cells stop growing receptors for them. So a few rounds of the drug could make someone less prone to allergies.

Designing a drug to specifically alter the shape of a molecule has never been done before. But, coincidentally, a drug called Xolair created by biotech company Genentech might do the trick. It is known to interfere with IgE, though no one quite knows why. 鈥淭his could be the mechanism by which it works,鈥� Sutton suggests.

Xolair has been promising in clinical trials, but the drug is a large molecule thathas to be administered by injection, which is very expensive. Sutton hopes that his team鈥檚 new understanding of IgE means they鈥檒l be able to design a drug that is a smaller molecule so you can pop it as a pill.

Unfortunately, not all allergies are triggered exclusively by IgEs. 鈥淎nti-IgEs are going to be useful, but not the cure for everything,鈥� cautions Hasan Arshad from the David Hide Asthma and Allergy Research Centre on the Isle of Wight, who worked on some of the initial studies with Xolair.