New Delhi, July 8 -- My one complaint comes back more than any other: the rash that went away is now back, with the same itch, sometimes in the same spot. I've tried the cream, finished the antihistamines, and switched my soap twice, and it still returns. Here is what my dermatologist usually tells me: "The allergy you keep treating may not be the real problem; the real problem is often the skin barrier underneath it."

Think of your skin's outer layer as a brick wall. The cells are the bricks, and the fats and oils between them are the mortar. When that wall is intact, it keeps irritants, allergens, and microbes out and water in. When the mortar breaks down, the wall gets leaky. Now, the things your skin would normally shrug off, dust, f...