Histamine is a molecule produced in the stored in the white bloods called eosinophils or mast cells and basophils. This chemical compound plays an important role in signaling the immune system to respond to invading pathogens or germs and foreign bodies. The Cleveland Clinic underlines that histamine is also responsible for allergic reactions when excess levels of it are secreted or the immune becomes oversensitive to this chemical. Histamine also functions as a neurotransmitter or chemical messenger in the brain and stimulates the production of gastric acid for digestion. Histamine causes several responses in the body, including allergic reactions, through its effects on various tissues.
Lung Smooth Muscles
Allergy-related asthma is triggered by overreactions by the immune system to increased secretions of histamine. The Department of Biology at Davidson College notes that patients with allergy-induced asthma are more sensitive to histamine-mediated bronchoconstriction or narrowing of the smooth muscle lining in the bronchioles of the lungs. Histamine causes contriction, narrowing, spasms and inflammation in these lung passageways, leading to asthma. Symptoms of asthma include coughing, wheezing, difficulty breathing and chest tightness.
Mucous Membranes
The Mayo Clinic notes that histamine triggers angiodema or swelling in the mucous membrane linings of the nose, mouth, throat, lungs and gastrointestinal tract. Swelling occurs because histamine dilates blood vessels, causing the leakage of fluids into these tissues. This also occurs just below the skin, causing allergic skin reactions such as swelling, hives and itching. In more severe allergic reactions, the swelling in the mouth, throat and lungs can restrict or even obstruct breathing.
Fluid Production
Histamine also increases production of mucus in the nasal passageways and lungs leading to a runny nose, nasal congestion and fluid in the lungs. The Pharmacology Corner explains that histamine can cause these secretions to be both thick and excessive. This occurs in allergic reactions because histamine activates both H1 and H2 receptors in the body that produce mucus to trap foreign particles that enter the body.
Stomach Tissue
Histamine also affects gastric or stomach secretions. The Pharmacology Corner explains that this mechanism involves the activation of H2 receptors in the stomach that secrete acid. For this reason, some medications that decrease acid secretion in the stomach to treat heartburn target the reaction between histamine and H2 receptors.
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