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"For the first two years of her life, my daughter, Katlyn, was knocking on heaven's door every day," says Daisy Demerchant, a 26-year-old mom living in Centreville, New Brunswick, just north of Maine. "Two months after she was born she started getting sick, and she never got better." At six months Katlyn was diagnosed with "bubble boy" disease, formally known as severe combined immunodeficiency (SCID), which robs the immune system of the ability to fight infection. There are many causes of this disorder; in Katlyn's case it was lack of the enzyme adenosine deaminase, or ADA, which rids the body of a natural toxin called deoxyadenosine. When the toxin builds up, it destroys T and B lymphocytes, the body's infection-fighting immune cells. As a result, Katlyn's immune cells were dying.
Treatment options ranged from risky to grim. One was a bone marrow transplant, in which imported donor cells could manufacture healthy T cells to fight invading germs. But bone marrow transplants can have lethal complications and often require drugs that further inhibit the patient's immune system, leaving a window of...