Bet v I allergen is a family of protein allergens. Allergies are hypersensitivity reactions of the immune system to specific substances called allergens (such as pollen, stings, drugs, or food) that, in most people, result in no symptoms. Trees within the order Fagales possess particularly potent allergens, e.g. the prototypical Bet v 1, the major white birch (Betula verrucosa) pollen antigen. Bet v 1 is the main cause of type I allergies observed in early spring. Type I, or immunoglobulin E-mediated (IgE-mediated) allergies affect 1 in 5 people in Europe and North America. Commonly observed symptoms are hay fever, dermatitis, asthma and, in severe cases, anaphylactic shock. First contact with these allergens results in sensitisation; subsequent contact produces a cross-linking reaction of IgE on mast cells and concomitant release of histamine. The inevitable symptoms of an allergic reaction ensue.