Immunotherapy—the injection of slowly increasing doses of materials to which we are allergic—is about a hundred years old. (It's sometimes also called hyposensitization or desensitization.) We are still not sure exactly how it works, but in the past few years the picture has been getting clearer. There is now increasing evidence that after exposure to many materials (called antigens) that are capable of stimulating the production of antibodies, the body will choose to follow one of two possible paths. One path leads to the production of antibodies that cause allergies. The other path leads, more happily, to the production of different kinds of antibodies that the body may use to prevent infection. How the antigen is administered (injected, for instance, instead of inhaled) helps determine which path the immune response will take. Immunotherapy takes advantage of this phenomenon to push the immune response toward the nonallergy path.