Histamine is an organic compound which is found mainly in your intestines, brain, spinal cord and uterus. It is a compound that is involved in the local immune response of our bodies. If you develop an allergy due to any source of foreign body, histamine is the main chemical compound that causes itching.
Histamine is found in your white blood cells- basophils and mast cells which attack pathogens that enter your blood circulation. Other than itching it also cause the dilatation and increased permeability of your small blood vessels( capillaries)
Histamines act in your body through several receptors called H1, H2,H3 and H4.
Mainly H1 receptors in the periphery causes itchiness, constriction of bronchus, blood vessel dilatation.
H2 receptors are important in producing hydrochloric acid(HCl) which modulate your stomach functions. H3 receptors also play a role in gastric acid secretion. H4 receptors are involved in hypersensitivity reactions after a foreign body enters your body through skin, orally or by any other route.
Histamine is a small compound compared to other biological compounds in our body. But it is involved in many physiological functions of the body.
Physiological functions that involve histamines
- Blood vessel dilatation and reducing blood pressure
Due to hyperpermeability and vasodilatation the pressure inside your blood vessels reduces drastically. In anaphylaxis it’s the main mechanism that results in drop in blood pressure. Intramuscular 1:1000adrenaline is administered immediately which helps to increase the blood pressure and recover a patient from anaphylactic shock.
- Increased secretion from nasal mucosa
Due to increased permeability of your blood vessels, secretions come out of your nasal mucosa. Histamine is the culprit for having a runny nose and watery eyes.
- Gastric acid secretion
The stomach acidity is important to digest the food and fight against pathogens that you ingest with food.
Histamine plays a key role in production of HCl which maintains the low pH of your stomach.
If you have gastric ulcer disease and chronic gastritis your doctor will prescribe histamine receptor blockers such as Omeprazole to reduce the gastric acid production.
- Sleep wake regulation
Histamines act on your hypothalamus and help to maintain the sleep- wake cycle.
- Sexual functions including erection
If you are on antihistamines it can cause decrease sexual desire and erectile dysfunction.
Antihistamines are medicines which are used to treat allergies. You can get them over the counter if you experience nasal congestion, sneezing or skin allergies caused by animal bites, pollen, dust and etc.
They simply act by blocking the histamine receptors of your body.
Examples – Cetirizine, Chlorpheniramine, Cimetidine, Famotadine
Antihistamines are used for symptom relief in variety of conditions including
- Itching and urticarial rash
- Allergic rhinitis- runny nose
- Food allergies and insect bites
Some antihistamines cause sedation where you feel increased sleepiness during the day time. But newer antihistamines are developed to overcome these side effects. Some non sedating antihistamines are Cetrazine and loratadine. It’s available as tablets and syrup. Usually a 10mg tablet once daily is prescribed by your doctor for allergic rhinitis, urticarial rash, insect bites and food allergies.
If you are on a sedating antihistamine such as promethazine, chlorpheniramine you have to be extremely careful of driving, swimming and operating machines. You have to avoid taking alcohol if you are on antihistamines.