The basic principle of immunity is to recognize and protect oneself and to recognize and reject foreign bodies. Therefore, immunity is an essential function for the survival and reproduction of an organism. It is thanks to normal immune function that an organism can maintain its own stability and reject foreign substances from outside (e.g. bacteria, viruses) or endogenous (e.g. cancer cells). Therefore, normal immune function protects the organism. Allergies, also known medically as allergic reactions, are an abnormal type of immune response. This reaction is usually an unusual immune response to substances that are harmless to the body (e.g. pollen, food, drugs), which leads to damage to the organism. Typical allergic diseases include: bronchial asthma, allergic rhinitis, allergic conjunctivitis, eczema, food allergy, drug allergy, insect allergy and anaphylaxis. The substances that can cause allergies are called allergens. Allergy can occur only when an atopic patient is stimulated by an allergen to which he is precisely allergic.