Histamine is a protein compound that plays a key role in protecting the body against foreign invaders, but if the immune system becomes sensitized to allergens, excess amounts of histamine can cause symptoms of an allergic reaction. The University of South Carolina Medical School explains that the immune system can be triggered to release histamine as a response to contact with allergens such as pollen, animal fur, dust mites, mold, insect bites, and certain foods and medications.
Asthma
Allergy-related asthma can be induced by histamine released into the bloodstream. This causes irritation, inflammation and swelling of the mucous membranes that line the air passages of the lungs, as well as increased spasms or contractions of the smooth muscle walls of the lungs. The University of Maryland Medical Center lists symptoms of allergy-related asthma such as wheezing, difficulty breathing and chest tightness.
Nasal Congestion
Nasal and sinus congestion is caused by histamine production as a response to inhaled allergens. Individuals with hay fever allergies have varying symptoms that include sneezing, runny nose, nasal congestion, nasal itching, sinus congestion and irritation, watery eyes and itchy ears. These symptoms occur because histamine dilates the blood vessels in the mucosal lining of the nose, sinuses and throat, causing fluid to accumulate and leak from the nose, eyes and throat.
Itching
Histamine also causes skin irritation that leads to itching, scratching and inflammation in response to an allergen. The Cleveland Clinic explains that skin itching symptoms may appear only on areas of the skin exposed to the allergen, certain areas or generalized itching on the whole body. Itching may also be a secondary symptom of other histamine-related effects such as eczema and contact dermatitis.
Hives
Hives, or urticara, are one of the most common symptoms of food, environmental and other allergies, notes MayoClinic.com. Hives are raised, red, itchy round, swollen areas on the skin caused by circulating levels of histamine in the blood. This temporary symptom typically appears on the face, neck, back, chest, stomach, arms and hands, and subsides completely within a few hours.
Swelling
Swelling, or angioedema, occurs in the mucous membrane linings of the body, including the mouth, throat, lungs and digestive tract as well as just below the skin. This symptom is triggered by histamine that causes dilation of blood vessels and the accumulation of fluids in these areas. Swelling due to an allergic reaction is typically noticeable around the face, lips, eyes, hands and feet. In more severe cases, swelling that occurs in the mouth, throat and lungs can make it difficult to breathe or even obstruct breathing.
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