Aller: An allergen is anything that triggers an organism's immune system to respond. In many cases, the allergens themselves aren't toxic, but they may trigger extreme responses by the immune system that lead to death. For example, some people with allergies to these allergens; this swollen airway, in turn, restrict their breathing and may cause death. Formaldehyde used in building materials is common allergen and results in sick building syndrome, which I describe in Chapter 15