Histamine is a naturally occurring organic compound within the body, a chemical messenger facilitating communication between cells. It is produced through the decarboxylation of the amino acid histidine, catalyzed by the enzyme L-histidine decarboxylase. Once formed, histamine is stored or rapidly inactivated, playing a role in many biological processes.
Histamine’s Diverse Roles in the Body
Beyond its common association with allergies, histamine performs diverse physiological functions. In the brain, it acts as a neurotransmitter, influencing the sleep-wake cycle, promoting wakefulness and suppressing REM sleep. Histamine also impacts appetite regulation, cognitive function, and memory.
In the gastrointestinal system, histamine is involved in stimulating gastric acid secretion, necessary for digestion. This action is mediated by histamine receptors in the stomach lining. Histamine also contributes to the immune response and inflammation, defending against foreign invaders.
Histamine is stored in specialized immune cells called mast cells and basophils, found in connective tissues like the skin, lungs, and intestines. When these cells detect a foreign substance, they release histamine and other chemicals. This release helps to increase the permeability of capillaries, allowing white blood cells and protective substances to reach affected tissues and combat pathogens.
Histamine and Allergic Responses
Histamine is widely recognized for its role in allergic reactions, where the immune system overreacts to harmless substances called allergens. When an individual is exposed to an allergen, immune cells called B-cells produce IgE antibodies. These IgE antibodies then attach to mast cells and basophils, sensitizing them to the specific allergen.
Upon subsequent exposure to the same allergen, the sensitized mast cells and basophils rapidly release stored histamine and other inflammatory chemicals. This sudden release triggers reactions leading to characteristic allergic symptoms. Histamine causes blood vessels to dilate and become more permeable, allowing fluid to leak into tissues.
Increased vascular permeability contributes to symptoms like swelling, watery eyes, and nasal congestion. It also stimulates nerve endings, causing itching and sneezing. In severe allergic reactions, especially food allergies, histamine can lead to vomiting, diarrhea, and lung muscle constriction, making breathing difficult.
Modulating Histamine Activity
The body has natural mechanisms to regulate histamine levels and activity. Two primary enzymes, diamine oxidase (DAO) and histamine N-methyltransferase (HNMT), are responsible for breaking down histamine. DAO primarily degrades extracellular histamine, while HNMT metabolizes histamine mainly in the central nervous system and other tissues.
When histamine levels are elevated or its effects become undesirable, medications known as antihistamines can be used to modulate its activity. Antihistamines work by blocking histamine from binding to its specific receptors on cells. There are four main types of histamine receptors in the body: H1, H2, H3, and H4.
H1 receptor antagonists are commonly used to alleviate allergy symptoms like itching, sneezing, and hives by blocking H1 receptors found on smooth muscle cells, nerve endings, and blood vessels. H2 receptor antagonists, on the other hand, target H2 receptors primarily located in the stomach, reducing gastric acid secretion and used for conditions like acid reflux. While H3 and H4 receptor antagonists are under investigation for various therapeutic uses, H1 and H2 blockers are widely available and prescribed.