Excess histamine release causes inflammation and allergy. They are chemicals released by specialized cells, called mast cells, as a defense mechanism. Mast cells also play a role in immunity. Histamines have an inflammatory effect on the body.
Itching and Pain
Excess histamine release causes itching and can also cause pain. Two types of histamine receptors that are known to cause specific effects are present in the body are H1 and H2. Itching is caused by activation and release of H1 receptors when the body is exposed to foreign substances that induce an allergic reaction. Because histamine (H1) receptors are abundant in the skin, exposure to an allergen causes their release, producing itching and pain from stimulation of sensory nerve endings. An example is itching, swelling and pain that occurs in response to an insect bite.
Swelling
Swelling is an effect of excess histamine release and happens when blood and fluid escape from the capillaries and leak into extracellular spaces. Blood vessels dilate and collapse, causing blood to become trapped and contributing to swelling, also known as edema. Anaphylactic shock is a severe type of allergic reaction that results from excessive release of histamine. Swelling in the throat, around the eyes, and lips, and wheezing and difficulty breathing are hallmark symptoms of anaphylaxis. The effect of histamine release on the lungs can lead to pulmonary edema, fluid that collects in the respiratory air sacs, from leakage of fluid through the veins in the lungs (pulmonary veins). The same mechanism is what causes runny nose, itchy eyes and sneezing in response to a cold virus or allergen.
Increased Gastric Secretions
Specialized cells in the stomach, called parietal cells, secrete hydrochloric acid. Too much hydrochloric acid leads to ulcers and inflammation. Mast cells are also present in the lining of the stomach that release histamine and contain H2 receptors. Histamine has an effect on parietal cells, combined with other enzymes, that leads to increased gastric secretions.
Sleep and Wakefulness
Histamine affects brain chemicals that regulate sleep and wakefulness. They act as neurotransmitters in the brain. Neurons in the brain that transmit signals contain histamine that increase when we are awake. The effect of histamine on wakefulness explains why antihistamine medications make us sleepy.
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