How Antihistamines Work

For millions of us allergy sufferers, antihistamines ease the annoying discomfort accompanying allergic reactions. Being an allergy sufferer myself, through the decades I have taken both over-the-counter and prescription antihistamines, some of which have been discontinued. Allergic reaction symptoms include nasal congestion, sneezing, coughing, headache, and itching indicating sensitization to certain allergens such as pollen, mold, dust, or insect venom. Repeated exposure results in allergic reactions due to antigen-antibody interactions.

First, antigens come into contact with lung, gastrointestinal tract, and/or skin tissue, and enter into the bloodstream. Next, mast cells and white blood cells misidentify these as invaders and inappropriately respond by releasing histamines causing tissue injury. The severity of the allergic reaction is directly proportional to the amount of histamine released.

Histamines dilate small blood vessels and capillaries, but contract smooth muscles. Responses include decreased blood pressure, increased heart rate, difficulty breathing, constipation or diarrhea, heartburn and nausea, and proteins and fluids leaking from capillaries. Nasal mucous membrane capillaries leaking, result in nasal congestion. Skin capillaries leaking produce hives and swelling resulting in pain and itching.

Thankfully all antihistamines block most of histamines effects by competing at histamine receptor sites thereby preventing them from producing an effect on the tissues. Antihistamine drugs prevent, but can’t reverse, histamine responses. Some of the general uses of antihistamines include relief from nausea and vomiting, relief from motion sickness, and relief from coughs. Usually antihistamines are administered orally since they are well absorbed in the intestinal tract, or topically, although a few can be given intravenously. To maintain a therapeutic dose, these medicines are given two to four times per day because the liver rapidly metabolizes them.

First generation antihistamines, which can be used interchangeably, include chloropheniramine (Chlortrimeton®), diphenhydramine (Benadryl®), and promethazine (Phenergan®). Some of the side effects include nose and mouth dryness, and drowsiness. Some antihistamines are also used as local anesthetics because they depress sensor nerve activity. When taking these antihistamines, it is advisable to not drive or perform hazardous tasks, and not to use alcohol or other drugs.

Second generation antihistamines include cetirizine (Zyrtec®), and Loratidine (Claritin®), and third generation antihistamines include fexofenadine (Allegra®), claim to be more selective for histamine receptors and cause less drying or sedating. However, these are contraindicated in patients with hepatic dysfunction, and when taking certain antifungal, antibiotic, and serotonin release inhibitors due to unfortunate deaths. My personal experience with these antihistamines is I did not find the relief I needed.

Because of the sedative effect, antihistamines are used in sleep aids like Nytol®, or Tylenol PM®. Because of the drying effect, antihistamines are found in over‑the‑counter skin ointments/sprays/creams, cold remedies, and cough syrups.

However, people with hypertension, cardiovascular disease, urinary retention, increased intraocular pressure, narrow-angular glaucoma, peptic ulcer, or prostatic hypertrophy should not use antihistamines. They are also contraindicated in dehydrated children, nursing mothers, newborn or premature infants.

Nonetheless, antihistamines are widely used successfully by millions. In conclusion, antihistamines work by providing us allergy sufferers much needed: relief.

©
Debby Bolen, RN

What is an allergy?

Many people think of allergies as nothing more than sniffles. However, nothing could be more far from the truth. According to the American Academy of Allergy, Asthma and Immunology, allergies are the sixth leading cause of chronic illness in the United States affecting the productivity and quality of life of 40 to 50 million people, or more than 20 percent of the population.

Actually, an allergy is an adverse immune system response induced by exposure to a substance, named an allergen, resulting in harmful tissue injury upon subsequent exposures. Normally immune systems identify foreign invaders and send white blood cells to destroy infections. Unfortunately for us allergy sufferers, our immune systems incorrectly identify nontoxic substances as invaders and our white blood cells overreact causing more damage to our bodies than invaders would. These inappropriate hypersensitive responses vary from sneezing, watery eyes, stuffy sinuses, coughing, wheezing, shortness of breath, itching, hives/skin rashes, headaches, and fatigue, to potentially life-threatening situations. In addition, secondary bacterial infections can set in including sinusitis, and ear infections.

Upon initial exposure to an allergen, symptoms may not even be experienced even though the immune system becomes sensitized to it. First, immune cells called macrophages engulf the allergen and link up with toxin-fighting white blood cells. Next, other white blood cells produce a protein called IgE, which is programmed to react to the allergen. Then, the allergen-specific IgE antibodies attach to mast cells in the digestive tract, skin, or in mucous membranes of the eyes, nose, throat, or upper airway. These cells store allergy-producing chemicals including histamines, which are responsible for the allergic reaction. When allergen particles fit in between two IgE proteins creating linked pairs, the mast cells break open and release histamine and other chemicals, such as eosinophils, causing inflammation, increased secretions, itching, or airway spasms.

Anything breathed in, eaten, or touched is a potential allergen. Common types of allergens include dust mites, pollens, metals, cosmetics, animal dander, latex, food, insect venom, medicine, food additives, chemicals, and mold. In addition, many sufferers react to multiple substances.

Allergy reactions are categorized as mild, moderate, or severe. Mild reactions affect a specific area of the body but do not spread and are similar to a cold, for example, watery eyes, and sneezing. However, moderate reaction symptoms spread to other body areas for instance a spreading rash or difficulty breathing. A severe allergic reaction is anaphylaxis which is a rare, life-threatening emergency affecting the entire body. Anaphylaxis progresses rapidly to serious upper airway swelling resulting in difficulty swallowing and breathing, accompanied by dizziness and mental confusion due to a rapid drop in blood pressure.

When allergy symptoms reoccur or occur longer than a couple weeks, make an appointment to be medically evaluated. Some sufferers obtain relief over longer periods of time with immunotherapy, named allergy shots. In conclusion, allergies cannot be prevented but are treated by making changes in your environment, and by taking over-the-counter or prescription medications such as antihistamines, decongestants, and nasal sprays.



©
Debby Bolen
Registered Nurse