Antihistamines counteract a natural chemical in the body that triggers allergic reactions. Available in various forms, including tablets, liquids, nasal sprays, and eye drops, antihistamines provide relief from common issues like sneezing, itching, and watery eyes.
Understanding Histamines and Allergic Responses
Histamines are signaling chemicals produced by the immune system, primarily known for their role in allergic responses, though they also regulate sleep and stomach acid. When an allergen, such as pollen, pet dander, or dust mites, enters the body, the immune system mistakenly identifies it as a threat. In response, immune cells, particularly mast cells, release large quantities of histamine into the surrounding tissues.
Once released, histamines bind to specific cell receptors, initiating reactions like blood vessel widening, increased permeability, and inflammation. The body’s attempt to expel the perceived threat results in common allergy symptoms, including sneezing, nasal congestion, runny nose, itchy or watery eyes, and skin rashes or hives.
How Antihistamines Block Reactions
Antihistamines work by targeting H1 receptors located on various cells throughout the body. These receptors are found on nerves, smooth muscle, glandular cells, and the lining of blood vessels. By blocking these H1 receptors, antihistamines prevent histamine from binding, stopping the chain of events that leads to allergic symptoms.
Essentially, antihistamines act as competitive blockers, occupying the H1 receptor sites so that histamine cannot attach and activate them. This mechanism reduces the inflammation, swelling, and increased mucus production characteristic of an allergic reaction. By preventing histamine from exerting its effects, these medications mitigate symptoms like itching, redness, and swelling.
First and Second Generation Antihistamines
Antihistamines are broadly categorized into two main generations, differing primarily in their side effect profiles and how they interact with the brain. First-generation antihistamines, such as diphenhydramine (commonly known as Benadryl), are older compounds. These medications can easily cross the blood-brain barrier. Their ability to enter the central nervous system allows them to affect H1 receptors in the brain, leading to common side effects like drowsiness and impaired cognitive function. First-generation antihistamines also have a shorter duration of action, typically requiring more frequent dosing.
In contrast, second-generation antihistamines, which include medications like loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra), are newer and designed to be less sedating. These newer antihistamines are less likely to cross the blood-brain barrier, meaning they primarily act on peripheral H1 receptors outside the brain. This selective action results in significantly less drowsiness and fewer central nervous system side effects compared to their first-generation counterparts. Second-generation antihistamines also offer a longer duration of action, often providing 12 to 24 hours of relief with a single dose.
Everyday Uses of Antihistamines
Beyond their primary role in treating allergies, antihistamines have several other common applications. Some first-generation antihistamines are used for short-term insomnia due to their sedative effects. These older medications can also be effective in preventing and treating motion sickness. This effect stems from their ability to block histamine and other chemicals in the brain’s vomiting center, reducing feelings of nausea and queasiness.
Antihistamines are also used to manage certain skin conditions that involve histamine release, such as hives (urticaria) and some forms of itchy eczema. Additionally, they can provide relief from the itching and swelling associated with insect bites and stings.