One of the more dangerous food allergies, peanuts are responsible for the majority severe food-related allergic reactions. They are one of eight foods that account for more than 90% of food allergies; the others are cow's milk, eggs, soy, wheat, nuts from trees (walnuts, hazelnuts, almonds, cashews, pecans, pistachios, etc.), fish, and shellfish.
Peanut butter was a staple in my childhood home, but times have changed: today many homes and schools are peanut-free zones. In part this is because the reaction to peanuts can be very severe, but it's also because most people believe that up to 30% of the populace is allergic to the little nut. Most people are mistaken: only 1-2% of adults and 4-8% of children are allergic to these groundnuts (which aren't actually nuts at all, but legumes).
Peanut allergies tend to present early in life - from 14 months to 2 years of age - usually at first exposure. The reaction is a shock known as anaphylaxis, which can be severe enough in some cases to cause death. Reactions are often caused by exposure to miniscule quantities of peanut - often less than a single nut - and highly sensitive people can react to truly tiny trace amounts. Sensitivity to peanuts may go away, or "resolve" in medical parlance, as the sufferer ages, though it may not. Some speculate that children gain hereditary sensitivity to peanuts - atropy - through consumption of breast milk, but this has not been proven.
Management of peanut allergy involves avoidance of contact with peanuts or foods containing any trace of the offending item. If accidental exposure does occur, epinephrine can be used, but it should be administered promptly, so the highly sensitive should carry a self-injection device with them at all times. Antihistamines can also be used, but epinephrine is more effective and should be used as first-line treatment.
For more information on peanut and other life-threatening allergies and their management, see www.anaphylaxis.org.
Information from "Peanut Allergy: An Overview" by Saleh Al-Muhsen, Ann E. Clarke, and Rhoda S. Kagan, in Canadian Medical Association Journal, May 13, 2003.