Histamine, Mast Cells, and Hay Fever Allergies
Linda Crampton is a writer and teacher with a first class honors degree in biology. She often writes about the scientific basis of disease.
The Problem With Histamine
The hay fever season is a miserable time of year for many allergy sufferers. The sneezing, runny nose and congestion of hay fever are not only very unpleasant but can also interfere with daily activities. The symptoms are caused by an abnormally high level of histamine in the blood and tissues. The histamine level increases due to a hypersensitive reaction to pollen grains, which act as an allergen. The disorder is also called seasonal allergic rhinitis.
Histamine is a normal component of the body and has useful functions. Unfortunately, in some situations it can cause problems. During the allergic response involved in hay fever, histamine is released from mast cells and causes the typical symptoms of a pollen allergy.
Medications such as antihistamines can help to relieve the symptoms of hay fever. Strategies to prevent the disorder from developing or to reduce its intensity can be very useful, however. These techniques can reduce the amount of medication that's needed or even eliminate the need for medication entirely.
What Are Mast Cells?
Mast cells are located in connective tissue. Connective tissue occurs in several different forms and has a wide range of functions, including joining organs and structures together and providing support and protection. The mast cells are particularly common in the connective tissue under the skin, around blood and lymphatic vessels, around the linings of the gastrointestinal tract and the air passages, and around the nerves.
Using a microscope, we can see that mast cells contain a large number of granules. These give the cells a speckled appearance. The granules store chemicals, including histamine. Histamine is produced from the amino acid histidine, which is obtained from our diet. When mast cells are activated, their granules disappear as they move to the surface of the cell and release their chemicals. This process is known as degranulation.
Mast Cell Activation
Mast cells have important functions in our immune system. They are involved in both wound healing and in defending the body against pathogens (microbes that cause disease). These are vital processes. Mast cells are also involved in the allergic response, which is a process that allergy sufferers would like to avoid.
When an allergen enters the body for the first time, it joins to a type of white blood cell called a B cell. The B cell then makes plasma cells. These in turn produce antibodies known as immunoglobulin E, or IgE. The antibodies bind to the membrane covering mast cells. The mast cells are now ready to be activated by a future exposure to the allergen.
When the body is exposed to the allergen for the second and subsequent times, the allergen binds to the antibodies on the mast cells. This activates the cells, causing them to release histamine as well as other chemicals. As a result, allergy symptoms appear.
How Allergies Develop: Additional Details
Histamine and Its Receptors
Histamine acts as a chemical messenger. It joins to receptors on the membrane of a cell in order to carry out its job. Four types of histamine receptors have been discovered so far: the H1, H2, H3, and H4 receptors. Interestingly, the response of different cells to histamine may not be identical, since it depends on which type of receptor has been activated.
Histamine Receptors and Their Major Effects
Smooth muscle, linings of blood vessels and airways
Contributes to the inflammatory response; causes an acute allergic response, vasodilation, and bronchoconstriction
Stomach acid secretion
Neurons in the brain
Mast cells and some types of white blood cells
Plays a role in the immune response
Inflammation and Allergies: H1 Receptors
Histamine joined to H1 receptors contributes to the body's inflammatory response. This occurs in response to an injury or an infection. The histamine also plays a role in the allergic response, which is an abnormal, hypersensitive response to a harmless substance.
The Inflammatory Response
The inflammatory response is a complex process that is produced through several mechanisms that work together, including the release of histamine. Inflammation is often unpleasant but can be useful. Extra blood flows to the injured area, producing redness and heat. The blood vessels dilate and become leaky, allowing fluid to leave the vessels and enter the tissues. This causes swelling in the tissues and may put pressure on nerves, producing pain. The leaked fluid contains materials that fight infections. White blood cells are also attracted to the area to help heal the injury. Inflammation in the short term is helpful, but if it's severe or lasts for too long it can be harmful.
The Allergic Response
The effects of an allergic response range from relatively minor (such as the symptoms of hay fever) to life threatening (such as the symptoms of anaphylaxis). Anaphylaxis is a dangerous, body-wide allergic response. Histamine triggers widening of blood vessels, which is known as vasodilation. It also causes the blood vessels to release fluid into the tissues. Vasodilation over a large area can cause dangerously low blood pressure. In sensitive people, histamine can also stimulate bronchoconstriction. This involves narrowing of the bronchi, the tubes that go to the lungs, which can cause difficulty in breathing.
Other Functions of Histamine
Stomach and H2 Receptors
In the stomach, histamine attached to H2 receptors stimulates the secretion of gastric acid (or stomach acid). This liquid is useful, since it activates an enzyme called pepsin which digests protein. It also kills some of the harmful bacteria that enter our digestive tract. Excess histamine causes the stomach to produce too much acid, however, which can be painful in someone with a stomach ulcer.
Brain and H3 Receptors
In the brain, histamine attaches to H3 receptors on neurons (nerve cells) and affects neurotransmission. This is the passing of the nerve impulse from one neuron to another over the tiny gap that exists between them. The process is controlled by means of chemicals called neurotransmitters.
When a nerve impulse reaches the end of a neuron, a neurotransmitter is released. The chemical travels across the gap between neurons and binds to a receptor on the next neuron, where it either stimulates or inhibits the cell.
Histamine acts as a neurotransmitter and also seems to modify the effect of other neurotransmitters. Histamine in the brain is released from neurons instead of mast cells.
Immune System and H4 Receptors
The H4 receptor is the most recently discovered histamine receptor. Histamine attached to H4 receptors seems to helps the immune system fight infections, but growing evidence suggests that it also plays a role in the allergic response.
Hay Fever and Its Symptoms
Hay fever or seasonal allergic rhinitis is an abnormal response to pollen from trees, grasses, and flowers. Occasionally, the technical term is also used to refer to a seasonal allergy caused by mold spores. The word "rhinitis" means inflammation of the nasal airways. The body overreacts to the presence of the pollen or mold and behaves as though it's an invader.
The symptoms of hay fever include a runny and stuffy nose, watery eyes, and a post nasal drip. The nose, throat or ears are often itchy as well. The eyes may be both itchy and red. In asthmatics, asthma may get worse. The person may suffer from wheezing, coughing, shortness of breath, and a tight feeling in the chest. Some people develop sinusitis (inflammation of the sinuses) or their sinusitis may get worse. People may also experience fatigue and have a general feeling of being unwell.
Hay fever isn't a serious condition, unless a person is an asthmatic, but it can certainly interfere with life as a person tries to deal with the symptoms. Someone with severe hay fever can have a miserable spring when the pollen first appears unless they find a way to control their allergy. In some people the symptoms appear in summer or even fall as new types of pollen are released. Luckily, there are things that can be done to reduce the severity of the symptoms or even eliminate them altogether.
Antihistamines and Other Medications
The most commonly used antihistamines block the H1 histamine receptors on cell membranes. This stops the histamine from attaching to the membrane and prevents it from doing its job. Antihistamines are sold in drug stores. Some types are available only by prescription, however. The newer antihistamines are less likely to cause drowsiness than the ones created in earlier times. The medications help with a runny nose, sneezing, and itching.
Decongestants help to relieve a stuffy nose. Once again, both over-the-counter and prescription versions are available. Prescription corticosteroid nose sprays reduce inflammation in the nose and cromolyn sodium prevents the release of histamine.
All medications—even ones bought without a prescription—can have side effects. It's important to read the information that accompanies a medication and to know what the side effects are. When a person first suspects that they have a seasonal allergy they should visit their doctor to get a diagnosis and a suggested treatment plan.
It's often most effective to start taking hay fever medications before the allergy season starts instead of waiting until the first symptoms occur. This shouldn't be done without a doctor's advice. Medications can be very helpful but should never be taken unnecessarily.
One Doctor's Advice About Hay Fever
Preventing or Reducing Hay Fever Symptoms
It's impossible to avoid all contact with the tiny pollen grains of a plant once they are released into the air. Luckily, a person can reduce their exposure to pollen by following certain procedures. Some of these steps may be helpful if you're having trouble controlling your hay fever symptoms. I follow many of them myself, since I have asthma and also experience hay fever.
- Don't go outside when the pollen count is high. Some TV or radio weather reports give the daily pollen count. You could also do a search for "pollen forecast" online to find a website that gives the pollen count for your area.
- Keep the windows and doors of your home closed when the pollen that you are allergic to is being released, if this is practical and doesn't cause the home to get too hot. An allergy test could be useful to find out what type of pollen you need to avoid.
- Vacuum regularly, using a vacuum with a HEPA filter. (A HEPA filter is a "high-efficiency particle arresting" filter.)
- Dust your home regularly with a damp cloth, which will reduce the spread of the pollen grains while you are dusting.
- Don't keep flowers inside your home.
- If you go outside in a car, keep the car windows closed and use an air conditioner to keep cool.
- Don't cut grass or walk in grassy areas during pollen season.
- Wear wrap-around sunglasses to prevent pollen from getting into your eyes.
- Shower and wash your clothes when you return home after being outside to remove pollen grains from your body and clothes.
- A common suggestion by allergy experts is to rub petroleum jelly at the base of each nostril to trap pollen on its way into the nose.
- Follow a "no smoking" rule in the home and avoid smoky areas when you are in other buildings, since smoke can irritate the airways and make allergies worse.
Taking steps to reduce pollen exposure improves my quality of life in the early spring, which is when my hay fever symptoms tend to appear. The attempt to prevent or at least reduce mast cell activation and histamine release is helpful for me.
Therearemanycausesofitchingwithoutarash.Someareminor; others are more serious. Both external and internal problems can cause itching. If the itching doesn't disappear soon, recurs after disappearing, is accompanied by other symptoms, or is hard to deal with, you should see a doctor for a diagnosis and treatment.