I found the experiment further down to explain the theory even better. > Some of the mice are ordinary, but others are not: using genetic engineering technique

I found the experiment further down to explain the theory even better. > Some of the mice are ordinary, but others are not: using genetic engineering techniques, Medzhitov’s team has removed the animals’ ability to make IgE. They can’t get allergies. > The animals may be spared the misery of hay fever caused by the ragweed pollen that will inevitably drift into their box on currents of air. But Medzhitov predicts they will be worse off for it. Unable to fight the pollen and other allergens, they will let these toxic molecules pass into their bodies, where they will damage organs and tissues. > “It’s never been done before, so we don’t know what the consequences will be,” says Medzhitov. But if his theory is right, the experiment will reveal the invisible shield that allergies provide us.