How Things Work: Antihistamines block allergy triggers

Credit: Juan Fernandez/Staff Credit: Juan Fernandez/Staff

The spring season brings blooming flowers, pleasant breezes, clear skies, and, unfortunately, allergies. Just thinking of allergies might induce mental pictures of stuffy noses, watery eyes, and sneezing. Usually, people will take a variety of medications for treating allergies, including steroids and allergy shots. One of the first medications typically given is an antihistamine, which works by blocking the effects of an immune system chemical in our bodies, fittingly called histamine.

Histamine is a molecule released in the immune system that leads to symptoms such as itching and runny nose. In general, when a person’s body comes in contact with an allergy trigger like pollen or ragweed, immune system cells called mast cells produce histamines. These molecules interact with receptors within the nose and throat, causing common allergy symptoms. They can also cause hives to form on the skin.

When any part of the body comes in contact with an allergy trigger, the body produces antibodies, which then interact with the histamine-producing mast cells. As a result, histamines are released and bind to special sites called receptors on cells in the nose and throat.

This interaction causes blood vessels to widen, which allows more blood to flow to the area where the interaction is taking place. It can also cause swelling, fluid leakage, and itching in the nose and eyes.

Antihistamines block the interaction of histamines with their receptors in the nose and throat, thus suppressing both the swelling and the widening of blood vessels.

The medications commonly achieve this by coating the receptors, preventing them from recognizing the histamines.

There are two general types of antihistamines. Sedating antihistamines can enter the brain and cause drowsiness. The most recently developed antihistamines, called non-sedating antihistamines, cannot enter the brain and therefore do not cause drowsiness. The sedating antihistamines are more commonly used since itching tends to worsen in the evening and the medicine can help the user fall asleep.

Once released by the immune system, histamines work very quickly. In fact, according to Discovery Health, “By the time your symptoms appear, the histamine has already attached to cell receptors, and the allergic reaction is well under way. That’s why you need to take antihistamines two to five hours before exposure to allergens.” This is because antihistamines generally work well only 15 to 30 minutes after they are taken and are most effective one to two hours after they are consumed.

Although antihistamines can be effective in preventing allergy symptoms, they also have a wide array of side effects such as drowsiness, headaches, blurred vision, and constipation. Moreover, antihistamines can interact with other medications and cause further problems, and therefore need to be taken with caution.

According to the health website, “Drinking alcohol with a sedating antihistamine can increase the drowsiness it causes and should be avoided. Also beware of a ‘hangover’ effect in the morning if the tablets are taken too late at night.”