What are Histamines?

Histamines are inflammatory mediators released by a variety of cells following exposure to certain antigens or irritants.

They are produced within  mast cells, neurons and basophils. and when released when activated. They

Histamine binds to target receptors in the nose, lung, skin, gastrointestinal tract, brain and near blood vessels via specific histamine receptors, especially H1 receptors.

They are capable of activating 4 receptors: H1, H2, H3 and H4.

The result is typically chemotaxis or a localized inflammatory response, much like what is seen when a red mark appears on the arm when scratched.

Once activated by an antigen, histamine causes a series of events leading to increased vascular permeability and dilation, nerve activation, and inflammatory cascades that are collectively produce symptoms of hypersensitivity including itching, sneezing, increased mucus secretion (i.e. rhinorrhea, etc.), bronchospasm and, if enough vascular tissue is involved, low blood pressure.

With the advent of selective antagonists or antihistamines, histamine binding to receptors are blocked and symptoms are supressed. It appears this is done by interacting with basophils, monocytes, macrophages, eosinophils and T Cells.

H1-receptors are blocked by medicines such as diphenhydramine (Benadryl), the effect is controlling the histamine mediated responses. In the various areas of the body, stimulation causes hives (skin rashes), broncho-constriction, motion sickness, blood vessels dilation, and smooth muscle relaxation and cellular permeability or widening of blood vessels leading to redness of the skin.  Excess activation of these receptors triggers the symptoms of hayfever and other seasonal allergies. H1 receptors are also located in the brain and are believed responsible for regulating sleep. This is why such anti-histamines cause drowsiness.

H2-receptors include the acid releasing parietal cells of the stomach. Medicines such as ranitidine and cimetidine (Zantac and Tagamet) stop the release of histamine induced acid secretion and are used to treat GERD, heartburn and gastric ulcers. THey are also present on neutrophils, the most common of the White Blood Cells. The effect of histamines here is in the inhibition of cytokine and antibody inflammatory reactions. They are also present in the heart and uterus.

H3-receptors are present in the brain cells and play a role in the release of serotonin, dopamine, Acetylcholine, norepinephrine and other neutrotransmitters. The implication of blocking over stimulation of these receptors is the supposed protection of the brain from neurological disorders including Alzheimers, Parkinsons, ADHD and schizophrenia, etc.  It is believed the H3 receptors are located on axon terminals by neurons, regulating the release of neurotransmitters when activated.

H4-receptors regulate the levels of white blood cell release from bone marrow. They have also been show to direct mast cells. They are located in the thymus, small intestine, spleen, the colon, bone marrow and basophils.

Within minutes of activation of cells that produce histamine, histamine is released and binds to target receptors in the nose, lung, skin, gastrointestinal tract and near blood vessels via specific histamine receptors, especially H1 receptors.

White blood cells, or leukocytes are a primary immune system mechanism that helps the body fight infections and other diseases.

There are several different types of white blood cells including:

  • Neutrophils - Roughly half of white blood cells are neutrophils. Neutrophils are usually the first cells of the immune system to respond in response to an invader such as a bacteria or a virus. As first responders, they also send out signals alerting other cells in the immune system to respond to the scene. They are teh primary cells of puss that is released from damaged tissue. Once released from the bone marrow these cells live for only around eight hours.
  • Eosinophils - Eosinophils also play an important role in fighting off bacteria and are very important in responding to infections with parasites. They are perhaps best known, however, for their role in allergy symptoms, when they essentially go overboard in mounting an immune response against something (like pollen) which it mistakenly believes is an invader. Only about 1% of total WBC, they are most concentrated in the digestive tract.
  • Basophils - Basophils, also accounting for only around 1 percent of white blood cells, are important in mounting a non-specific immune response to pathogens. These cells are perhaps best known for their role in asthma. When stimulated these cells release histamine and heparin among other chemicals. The products can result in the inflammation and bronchoconstriction in the airways.
  • Lymphocytes (B lymphocytes and T lymphocytes) - Lymphocytes are also very important in the immune system, with T cells being responsible for directly killing many foreign invaders. B lymphocytes (B cells), in contrast to the other types of white blood cells, are responsible for humoral immunity (in contrast to the non-specific immunity of other white blood cells.) They produce the antibodies that "remember" an infection and stand ready in case your body should be exposed.
  • Monocytes - Monocytes make up about 5% of the total white blood cells. Their primary function is to 'clean up' dead cells in the body.  garbage trucks of the immune system.

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