Antihistamines are medications used to relieve symptoms of allergic reactions. They work by counteracting the effects of histamine, a naturally occurring substance in the body.
Histamine’s Role in Allergic Reactions
Histamine is a chemical produced and stored in immune cells like mast cells and basophils. It has functions such as regulating sleep, aiding digestion, and acting as a neurotransmitter. When the immune system encounters an allergen, such as pollen, it triggers histamine release from these cells.
This release is part of the body’s natural defense. However, in allergic individuals, this response is excessive, leading to symptoms like itching, sneezing, watery eyes, and a runny nose. These are direct results of histamine’s action on tissues.
How Antihistamines Block Histamine
Antihistamines target specific H1 histamine receptors involved in allergic responses. They do not stop histamine production but prevent it from binding to its receptors. Imagine histamine as a key and the receptor as a lock; an antihistamine acts like a duplicate key, occupying the space. By blocking these sites, antihistamines prevent histamine from triggering allergic symptoms. This competitive binding reduces histamine’s effects. For example, blocking H1 receptors in nasal passages can alleviate sneezing and a runny nose.
Varieties and Applications of Antihistamines
Antihistamines are categorized into generations based on their chemical structure and sedative effects.
First-Generation Antihistamines
First-generation antihistamines, like diphenhydramine, cross the blood-brain barrier, causing drowsiness. These older medications treat acute allergy symptoms, insomnia, or motion sickness due to their sedative properties.
Second-Generation Antihistamines
Second-generation antihistamines, including loratadine and cetirizine, were developed to be less sedating. These newer formulations do not cross the blood-brain barrier as readily, resulting in fewer central nervous system side effects. Both generations treat seasonal and perennial allergies, hives, and itching from insect bites.
Understanding Side Effects and Precautions
Side effects vary by antihistamine type. First-generation antihistamines commonly cause drowsiness, dry mouth, blurred vision, and constipation. These effects can impair driving or operating machinery. Second-generation antihistamines have a milder side effect profile, with drowsiness being less common. Some may experience mild fatigue or headache. Avoid alcohol with sedating antihistamines, as it amplifies effects. Individuals with glaucoma, prostate enlargement, or those pregnant or breastfeeding should consult a healthcare professional before use.