Sensitisation, or allergy, is an excessive, abnormal reaction of the immune system to substances in the environment that are foreign to the body, known as allergens, which it should normally ignore. In healthy people, these substances do not cause any discomfort, but in allergic people they cause a range of unpleasant and sometimes even dangerous reactions. Allergic diseases are widespread. In developed countries, various types of allergies affect approximately 30% of the population. Allergic symptoms can occur at any age. However, a certain correlation between age and type of disease can be demonstrated. Food allergies most often manifest themselves in infancy, hay fever, allergic conjunctivitis and atopic asthma in children and adolescents, and urticaria and sensitisation to hymenopteran insect venoms in adults.