H1 Antagonist: Definition & Side Effects

Instructor: Rachel Torrens

Rachel is a Nurse Practitioner with experience working as a high school teacher, skin surgery center, and as a family NP.

You've heard of antihistamines, but what about H1-receptor antagonist? They are actually one in the same! By reviewing the mechanism of action and side effects, you'll gain a firm understanding of the role antihistamines play in treating allergy-related maladies.

What Exactly is Histamine?

Spring has sprung, and Stan is excited! He grabs his ball cap and bat and heads outside. Within minutes, Stan begins sneezing. Then his nose starts to drip. Sadly, he heads back inside. Luckily, Stan's mother looks him over and declares 'Your allergies are the same as your father's!' She gives him some medication. Within thirty minutes Stan is happily outside again, unaffected by the environment.

This is a quick story, but it contains a vast amount of information about the human body's built-in defenses and the engineering marvels of modern medicine. First, let's look at what happened to Stan when he went outdoors.

When outdoors, pollen may trigger the release of histamine, which leads to sneezing, itching eyes, and runny nose.
Boy playing outside

When an allergen, such as pollen, enters the body, mast cells are notified of the intrusion. In response, they release chemical messengers to alert the body of this breach in defense. One of the chemical messengers is histamine. In an effort to rid your body of the irritant, histamine prompts your body to make more mucus, increase blood flow to the affected area and increase tear production. Most allergy symptoms are the direct result of histamine.

Histamine also causes allergy responses seen on the skin, such as the rash from poison ivy or the red, itchy bump that forms after a bug bites your skin. Now that you have an understanding of histamine and the various symptoms it can produce, let's move on to exactly how allergy medications, specifically antihistamines, work.

How an Antihistamine Works

When Stan's mother noticed his allergy symptoms, what did she do? She gave him a medication and within a short period of time he was able to play outside again. How is this possible?

Well, in order for histamine to trigger all those symptoms it must first bind to a cell's receptor. What if histamine arrived at the cell only to find its receptor site already filled, and thus was blocked from working? That's exactly what happened inside Stan.

He swallowed the medication, it was distributed to his cells, and bound to the histamine receptors. So when Stan went back outside, even if his body released more histamine it would be unable to bind to the cells as the receptors are already filled with the medicine. And voila! No symptoms. This is why these medications are called antihistamines because they block histamine from working.

This is where things get a little complicated. In fact, different cells have different types of histamine receptors. These various receptors are labeled H1 through H4. When histamines bind to cells with H1-receptors, allergy symptoms are triggered. Binding of histamine to the other three receptors affects different parts of your body, including stomach acid production, heart rhythm, and brain chemical production.

Naturally, when scientists were engineering a medication they only wanted to prevent the allergy symptoms, so they made a the drug would only bind H1-receptors. None of the other histamine receptors are hindered by antihistamines. For this reason, although commonly referred to as antihistamines, the technical name for these medications is H1-receptor antagonist. Again, these medicines block or antagonize the H1-receptors, preventing histamine from binding and causing allergy symptoms.

Generational Side Effects

Medications, once produced, are constantly being improved as researchers attempt to increase efficacy and decrease side effects. The different refinements in a class of drugs are referred to as 'generations'. The original antihistamines are referred to as first-generation antihistamines. Once improvements were made, scientists released second-generation antihistamines.

First Generation H1- Receptor Antagonist Medications

When first introduced to the market, this class of medications was revolutionary for people who suffered from allergy symptoms. They were able to enjoy the great outdoors without having to pack a box of tissues. However, this first generation of allergy medications had, and still has, more significant side effects.

Major side effects include:

  • Drowsiness
  • Dry mouth
  • Dizziness
  • Constipation or difficulty urinating
  • Stomach upset

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