Allergy is an abnormal reaction or increased sensitivity to certain substances. The allergic individual produces symptoms when exposed to these substances, which are harmless to non-allergic people. The main reason for this is that allergic people make a special type of antibody called immunoglobin E (IgE), which can react with environmental substances in harmful way. These substances which lead to secretion of IgE antibodies are called allergens.
Seasonal allergies like hay fever are more common in the spring when plant pollination begins. Trees and grass cause most symptoms in the spring, while ragweed is responsible for the majority of autumn symptoms.
Perennial allergies are usually year-round problems. Common triggers for perennial allergies include mould spores, pet dander and dust mite.
The most common allergens are animal dander, pollens, house dust, house mites, moulds, some drugs, and many food stuffs, especially fish, eggs, milk, and nuts. Bee and wasp stings may cause allergic reactions, with fatal results in rare cases. Additionally, feathers, wood, dyes, cosmetics, and perfumes may act as allergen.