What is an Antihistamine Definition Types Mechanism of Action and Uses
Antihistamines are a class of drugs that are used to cure the allergies triggered by the physiological action of the histamines. Histamines are the organic compounds that are involved in the local immune responses and act as a neurotransmitter. Histamines take part in the inflammatory response and act as a mediator of pruritus. Basophils produce histamines, and they are found in the connected nearby tissues. Histamines increase the permeability of the white blood cells, and this makes the pathogens to get more engaged with the infected tissues. Antihistamine examples are ranitidine, loratadine, meclizine, cetirizine, etc.
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The Action of Antihistamines on the Human Body, How do they Suppress Antihistamines?
Our immune system might start to mistake a harmless substance as a dangerous material or pathogen, this shows the start of the development of an allergy. These harmless foreign substances that are considered dangerous by the immune system, are named as the allergen. One of the most common allergens are pollen and milk (Lactose intolerance).
When the body comes in contact with any of the allergen, the immune system responds harshly in order to suppress the upcoming foreign substance named allergen. To deal with the allergens the immune system releases various chemicals and antibodies. One such chemical is named histamine, the histamine is the chemical which transports the information regarding the allergen to the brain and thus causing the major symptoms of the allergies. These symptoms can be nasal swelling, runny eyes, and, in some cases, mouth itching.
Antihistamines, as the name suggests (Anti-histamine), are the chemical that stalls and suppresses the action of the histamines. Thus protecting them from the various symptoms of the allergies. Antihistamine fights for a space in the neural receptors or the binding site of neural receptors with the histamine and thus stopping them from sending the information regarding the allergy to the brain.
However, Antihistamine has a limited range of applications and can not be used to treat many types of symptoms like for the symptoms of nasal congestion and swelling, the doctors do recommend the use of decongestant rather than Antihistamine. In many conditions, Antihistamine and decongestant are prescribed together along with some other types of medication depending on the patients.
Antihistamine Drugs Classification
Antihistamines drugs can be classified into two types according to the H receptor-targeted: -
H1: They are generally used to treat allergic reactions as well as mast cell-mediated disorders. This category can be further divided into two classes: first-generation H1 antihistamines, which have a central effect and are used as sedatives; and second-generation H1 antihistamine which have a lesser central effect and are used as antiallergic drugs.
H2: They are used for gastric reflux disease as they help in reducing the production of stomach acid by reversibly blocking the H2 histamines receptors in the parietal cells of the gastric mucosa.
Example of Drugs and Antihistamine Uses According to Antihistamine Classification
H1 First-Generation: Meclizine, Clemastine, Hydroxyzine, Brompheniramine, Dimetindene, Doxylamine, etc. that are used as sedative agents, antiallergic agents, to cure motion sickness, and an antiemetic agent. These drugs have strong sedative action and anticholinergic side effects.
H1 Second-Generation: Loratadine, Cetirizine, levocetirizine, azelastine, fexofenadine, etc. that are used as antiallergic agents, or adjuvant treatment for anaphylactic shock. These drugs are non-sedative or mildly sedative.
H2: Ranitidine, Cimetidine, Famotidine, etc. that are used to reduce the production of stomach acid and are generally used as a second-line treatment or in combination with proton pump inhibitors (PPIs).
Antihistamine Side-Effects
The severity and the frequency of the adverse effects of histamines vary according to the type of antihistamine. The H1 first-generation antihistamines are considered to be more harmful, and they usually cause drowsiness. H1 antihistamines cause anticholinergic effects like dry mouth and eyes, dizziness, urinary retention, mydriasis tinnitus, and tachycardia along with headaches. Benadryl and Chlor-Trimeton are examples of such drugs. The side effects of H2 antihistamines include confusion, dizziness, headaches. For example, Allegra, Clarinex, and Zyrtec. Cimetidine can lead to erectile dysfunction and gynecomastia because of its antiandrogenic effect. Cimetidine and ranitidine decrease the renal excretion of creatinine.
Solved Questions
Question 1. What is the difference between Claritin and Clarinex?
Answer: Claritin is loratadine that is metabolized in the liver to produce Clarinex that is desloratadine.
Question 2. How do antihistamines protect against allergies?
Answer: Antihistamines block the action of the histamines that gets triggered when our body comes in contact with components like pollen, dust mites, etc. These components make the histamines come in action to cause allergies like swelling of the nose, running nose and eyes, etc. Antihistamines block the course of action of histamines and protect us from allergies.
Question 3. Can antihistamines be used to cure skin allergies?
Answer: Yes, antihistamines can be used to take care of insect bites and skin allergies. Zyrtec and Claritin are both best antihistamines to help you with your skin allergies.
Question 4. What is an allergy?
Answer: An allergy is an immunological hypersensitivity that is mediated by immunoglobulin E antibody (IgE). An allergy is not related to any disease or infections.
Question 5. What causes an allergy?
Answer: Allergies generally occur when your immune system reacts to a foreign substance like pollens, bee venom etc. Allergies are basically the reactions of your body coming into contact with foreign substances.
FAQs on Antihistamine in Chemistry and Pharmacology
1. What is an antihistamine in chemistry?
An antihistamine is a chemical compound that blocks the action of histamine at histamine receptors, especially the H1 receptor, to reduce allergy symptoms. In medicinal chemistry, antihistamines are typically organic molecules containing:
- An aromatic ring system (often two phenyl rings)
- A short alkyl chain linker
- A terminal amine group (–NH or –NR2)
2. What is the chemical formula of common antihistamines?
The chemical formula of an antihistamine depends on the specific compound, for example diphenhydramine has the formula C17H21NO. Other common antihistamines include:
- Loratadine: C22H23ClN2O2
- Cetirizine: C21H25ClN2O3
- Chlorpheniramine: C16H19ClN2
3. How do antihistamines work at the molecular level?
Antihistamines work by acting as competitive antagonists at histamine receptors, especially H1 receptors. At the molecular level:
- Histamine binds to the H1 receptor and activates it.
- An antihistamine molecule binds to the same receptor site.
- This prevents histamine from attaching and triggering a biological response.
4. What are the types of antihistamines?
The two main types of antihistamines are first-generation and second-generation antihistamines. They differ in chemical structure and ability to cross the blood–brain barrier:
- First-generation: e.g., diphenhydramine (C17H21NO); more lipophilic and cause drowsiness.
- Second-generation: e.g., loratadine (C22H23ClN2O2); more polar and less sedating.
5. What functional groups are present in antihistamines?
Most antihistamines contain an amine group, aromatic rings, and sometimes ether or halogen substituents. Common functional groups include:
- Tertiary amine (–NR3): responsible for receptor binding.
- Aromatic phenyl rings: provide hydrophobic interactions.
- Ether group (–O–): present in diphenhydramine.
- Halogen atoms (Cl): found in chlorpheniramine and loratadine.
6. Why do some antihistamines cause drowsiness?
Some antihistamines cause drowsiness because they are lipophilic and cross the blood–brain barrier, blocking H1 receptors in the brain. First-generation antihistamines:
- Have lower polarity.
- Lack strong polar substituents.
- Easily penetrate the central nervous system.
7. What is the difference between first-generation and second-generation antihistamines?
The main difference between first- and second-generation antihistamines is their chemical polarity and selectivity, which affects sedation. Key differences include:
- First-generation: more lipophilic, cross the blood–brain barrier, cause sedation.
- Second-generation: more polar, less brain penetration, minimal sedation.
- Second-generation drugs are more selective for peripheral H1 receptors.
8. Are antihistamines acids or bases?
Most antihistamines are weak organic bases because they contain a basic amine group. The tertiary amine:
- Can accept a proton (H+)
- Forms water-soluble salts such as hydrochloride salts
9. How are antihistamines structurally related to histamine?
Antihistamines are structurally designed to mimic key features of histamine while preventing receptor activation. Histamine has the formula C5H9N3 and contains:
- An imidazole ring
- An ethylamine side chain
10. What are antihistamines used for in medicine?
Antihistamines are used to treat allergic reactions by blocking H1 receptor-mediated effects of histamine. Common uses include:
- Allergic rhinitis (hay fever)
- Urticaria (hives)
- Itching and skin rashes
- Motion sickness (some first-generation drugs)
