Histamine represents a compound derived from a decarboxylation of histidine. The chemical is attributed to allergic responses. Allergic reaction is triggered by the immunity in response to different harmless substances, including pollen, mites, dust, etc. The immune system recognizes those substances as potentially harmful invaders, and induces the release of antibodies to combat them. This process initiates the production of histamine, the substance provoking the inflammatory reaction of the body. Substances that may trigger allergic reactions are called allergens. Histamine is produced by basophils and the mast cells during allergic responses. Histamine affects different body systems. In fact, the body has several histamine receptors. Generally, histamine causes the inflammatory reaction of the body dilating the vessels of blood. The permeability of blood vessels gets high, allowing fluids to enter your skin and induce swelling. Blood vessels dilation can cause redness as well. During allergic responses the blood vessels become dilated and the white blood cells get access to foreign substances. Histamine reaction also causes the smooth muscles contraction, and the smooth muscle is responsible for inducing the attacks of asthma. Actually, anaphylaxis is a form of histamine reaction. This is a very severe condition which can trigger breathing difficulty and drop in the blood pressure. H2 histamine receptors are found in the gastric mucosa. If histamine stimulates these receptors, release of gastric acid raises causing irritation of the digestive system. Causes Histamine reaction can be induced by numerous allergens. More often, the immune reaction can be caused by dust, mites, animal dander, pollen, and mold. Several food types can trigger histamine reaction too. Insect stings or bites, in particular from wasps, fire ants, hornets and bees can cause an allergic response. Symptoms Histamine reaction can provoke several manifestations by irritating the H2 and H1 receptors. If it irritates the H1 receptors, a person will have hives, skin rash, itching, headaches, respiratory reactions and swelling. If histamine stimulates the H2 receptors, this evolves gastric histamine response related to the overproduction of gastric acid. Histamine affects several body systems. Skin rash and sensitivity, hives and itching, swelling, watery eyes and runny nose are typical symptoms for mild histamine reactions. If the smooth muscles are affected, the sufferer will experience difficulty breathing because of airways obstruction. In serious cases, histamine reaction can be life-threatening. Anaphylaxis is an allergic reaction, characterized by the swelling and constriction of airways, which leads to severe wheezing and shortness of breath. Other manifestations of anaphylaxis are vomiting and diarrhea, headache, confusion and dizziness, sudden and extreme drop in the blood pressure.