Bronchospasms
Histamine may produce bronchoconstriction either directly or indirectly. Directly, it may cause bronchoconstriction by activating the H1-receptor in airway smooth muscle. Indirectly, it may cause bronchoconstriction by stimulating afferent vagal fibers in the airways. A condition known as bronchospasm is characterized by a constriction of the muscles lining the airways (bronchi) in the lungs. When these muscles contract, it causes a narrowing of the airways. In general, bronchoconstriction is mediated by H1-receptors, the predominant type in the airways of the majority of animals, whereas H2-receptors mediate bronchodilation.
Because H1 receptors for bronchoconstriction are so prevalent in human airways, histamine acts as a potent bronchoconstrictor in humans, especially in asthmatic patients. Therefore, histamine intolerance typically manifests as an increase in histamine levels, which can directly impact the airways and result in a more severe symptom known as bronchospasm. When the airways get narrowed, they restrict the air entering or leaving the lungs. Because of this, the amount of oxygen that can enter the circulation and carbon dioxide that can leave the blood is reduced.
