Justine was startled: she had never told him that. “I don’t know where he comes from,” she said. “He is very focused on helping other kids through his hardship.”

Justine, meanwhile, never doubted her choice to be in the trial. In an e-mail, she wrote: “And Kari — as always — will always be there for him. She is our godsend. Our angel. I don’t have words to describe how I feel about her. . . . This has been the hardest journey of our lives, and she has saved my boy. Many times.”

Last month, Jack’s skin tested negative, and he is no longer allergic to either cashew or pistachios. (Treating cashew also treats pistachio, because the same protein causes both allergies.) He has gained a little weight and grown a small but significant amount, perhaps because his asthma is now moderate rather than severe, which Nadeau says may be a result of his participation in the Xolair trial. But he has to be careful; once he had an anaphylactic episode at school, and if he exerts himself within two hours before or after his dose, he can have a bad reaction. He is currently working on a cookbook to give to kids in the trial of all the ways to disguise disgusting nuts: in sushi, inside an olive or, his favorite, covered with chocolate and mixed into ice cream.

Just before Christmas, our son, Kieran, reached his first — and, for us, his most important — milestone. Nine months after we began, he ate 300 milligrams of each of his allergens (the equivalent of more than a full peanut, almond, cashew and hazelnut, as well as a bit of egg) and is no longer allergic to trace contaminants. Although it will take at least another year before he can eat a whole serving, he had become 300 times less allergic than he was at the start of the trial. When Violet’s peanut allergy is treated, our family can take down the sign on our front door forbidding anything made with eggs or nuts and welcome Girl Scout Cookies.

After Kieran’s updose in the hospital that day, we went down to the cafe to celebrate. We sat near the hospital’s glittering Christmas tree, listening to piano music, and I bought a turkey sandwich for us to share. For the first time in his life, I didn’t have to worry whether the bread had been cut with a knife or made on a surface that had also been used for mayonnaise or peanut butter. Kieran liked making a game of sharing the sandwich — a bite for him and a bite for me — but then he said, “I wish it all mine,” and when I told him he could have it, he slid off my lap and hopped up and down with joy.

The current studies at Stanford are in the analysis phase. The next and final phase of Nadeau’s multiallergy trial, which is in development, will very likely involve more patients and take place at multiple allergy centers across the country. Then, if that is successful and the treatment is approved by the F.D.A., it may eventually become standard. Research in Nadeau’s lab has found that treating allergies actually changes the genes by epigenetics (a chemical modification of the genes that does not affect the actual DNA sequence), so desensitizing children may reduce the likelihood that they will pass on the disease to their children. One day, perhaps, fatal anaphylaxis may become a sorrow associated with an earlier age, like dying of appendicitis or polio.

I sometimes have an image of Kari Nadeau taking Kieran and Jack and Tessa and the other children by the hand and guiding them step by step, dose by dose, away from the grave. Of course, none of us will ever know which, if any, of our children would have slipped over the edge, but we do know the treatment rescued us from a lifetime of peering into the abyss.