Allergies are hypersensitivity reactions of the immune system to actually harmless stimuli from the environment. Possible allergy triggers (allergens) are pollen, animal hair or food. A distinction is made between 4 different types of allergens: Inhalation allergens (e.g. pollen from trees, grasses, herbs but also house dust mite, animals with hair (but also saliva or glands can trigger allergies) as well as molds. Food allergens: Plant allergens (such as fruits, nuts, ...) and animal allergens (milk, eggs, fish, ...). Injection allergens (injectable drugs or venom from stinging insects (bee, wasp, ...) and contact allergens (jewelry: nickel, drugs (creams/ointments), cosmetics, fragrances, ...). Here you can find more information about allergens and what they actually mean.