Since the synthesis of histamine in 1907, new reports about its properties and role in the human body have periodically appeared. Its blood levels remain constantly low, almost undetectable, under normal physiological conditions. Increasing histamine levels lead to serious clinical symptoms, including cardiac arrest. Histamine is involved in many processes and allergic reactions. It exerts its effects by acting on receptors, of which four types have been described so far. They are found in numerous tissues, both those in direct contact with the external environment, such as the skin, gastrointestinal and respiratory mucous membranes, as well as those located in internal organs, such as the heart, the brain and on immune cells. The word “antihistamines” is most often used to refer to agents that block the type 1 receptor. These are classified as firstand second-generation antihistamines. The first generation includes non-selective hydrophobic substances that can cross the blood–brain barrier and affect many other types of receptors. Second-generation antihistamines are selective hydrophilic preparations with a long half-life, which means they can be used once daily. Due to their better safety profile, secondgeneration antihistamines are the treatment of choice in the chronic management of allergic diseases, such as allergic rhinitis and urticaria. First-generation drugs are only for short-term use.

histamine, antihistamines, first-generation antihistamines, second-generation antihistamines