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Antihistamines are a type of medication used to treat allergies caused by the physiological action of histamines. Histamines are organic compounds that play a role in local immune responses as well as acting as a neurotransmitter.
- Antihistamines is an inexpensive drug that can be brought without a prescription.
- It provides relief from sneezing, nasal congestion and hives.
- Histamines play a role in the inflammatory response and act as a pruritic mediator.
- It is produced by basophils and is found in the tissues that are connected to them.
- Antihistamines are used for short-term purposes.
- It increases the permeability of white blood cells, allowing pathogens to become more involved with the infected tissues.
- Ranitidine, loratadine, meclizine, and cetirizine are examples of antihistamines.
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Key Terms: Antihistamines, Histamines, Allergens, Nasal Congestion, Allergic Reaction, Brompheniramine, Terfenadine, White Blood Cells
Classification of Antihistamines
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Antihistamines are classified into two types:
First Generation Antihistamine (H1 Antihistamines)
H1 Antihistamines is a type of function that is formed by binding of H1 receptors of histamines in the smooth cell, mast cell and endothelial cells in the body. It is also connected with the tuberomammillary nucleus found inside the brain.
- H1 Antihistamines are also known as H1 blockers.
- It is a type of medication that inhibits histamine action at the H1 receptor, thereby alleviating allergic reactions.
Second Generation of Antihistamine (H2 Antihistamines)
In general, they do not cause sedation and drying those first-generation antihistamines do. H2 Antihistamine inhibit the activity of the H2 receptor which are most commonly found in the gastric mucosa parietal cells. It will induce the secretion of acid and help reduce gastroesophageal reflux disorder.
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Action of Antihistamines on allergies
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The action of antihistamines on the treatment of allergies is as follows:
- When our bodies come in contact with allergens such as pollen, dust mites, and pet dander, a chemical called histamine is produced.
- The presence of these compounds causes an allergic reaction.
- This allergic reaction causes nasal swelling, runny eyes, and, in some cases, mouth itching.
- Antihistamines inhibit the action of histamines, thereby protecting us from allergy symptoms.
- They work to alleviate the symptoms of various allergies, such as hay fever and food allergies.
- However, their application is limited because they cannot be used to treat all types of symptoms.
- If we have nasal congestion, the doctor will recommend that we take a decongestant.
- Some medications are created by combining antihistamines and decongestants.
- Antihistamines are used to treat any type of allergy.
- Brompheniramine (Dimetapp) and terfenadine (Seldane) are antihistamines that are synthetic in nature.
- These drugs actually interfere with histamines' natural action.
- They compete with histamines for receptor binding sites, which histamines use to exert their effect.
Antihistamines
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Examples of Antihistamine Drugs
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Some common examples of antihistamine drugs for first-generation and second-generation are as follows:
Drugs for H1 Antihistamines
Some of the drugs for H1 Antihistamines are as follows:
- Meclizine
- Clemastine
- Hydroxyzine
- Brompheniramine
- Dimetindene
- Doxylamine
These medications have a strong sedative effect as well as anticholinergic side effects.
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Drugs for H2 Antihistamine
Some of the drugs for H2 Antihistamine are as follows:
- Loratadine
- Cetirizine
- Levocetirizine
- Azelastine
- Fexofenadine
These medications are either non-sedative or mildly sedative.
Some other drugs for H2 Antihistamines for stomach problems are as follows:
- Ranitidine
- Cimetidine
- Famotidine
It is used as second-line treatment or in combination with proton pump inhibitors (PPIs).
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Side Effects of Antihistamine
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The severity and frequency of histamine side effects vary depending on the type of antihistamine. Some of the harmful side effects of antihistamines are as follows:
- H1 first-generation antihistamines are thought to be more dangerous, and they typically cause drowsiness.
- Anticholinergic effects of H1 antihistamines include dry mouth and eyes.
- People experience dizziness, urinary retention, mydriasis, tinnitus, and tachycardia, as well as headaches.
- H2 antihistamines can cause confusion, dizziness, and headaches.
- Allegra, Clarinex, and Zyrtec are a few examples.
- Cimetidine can cause erectile dysfunction and gynecomastia.
- Ranitidine reduces creatinine excretion in the kidney.
- The older groups of antihistamines, known as the first generation, are more dangerous, for example, Benadryl and Chlor-Trimeton.
- These medicines are particularly sedative.
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Things to Remember
- Antihistamine is a type of drug that is used for the treatment of allergies.
- It mainly works to block histamines in the body.
- Histamine is a compound that basically releases sneezing, itching, and congestion.
- It has the ability to prevent nausea, vomiting, and sedation.
- Antihistamines are a safer touch for pregnancies and breastfeeding.
- They do not harm the fetus and are also safe for children to use.
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Sample Questions
Ques: What is the most effective antihistamine to use? (2 marks)
Ans: Over-the-counter antihistamines such as Claritin and Zyrtec are widely used. Doctors regard them as safe and effective allergy treatments. Both are second-generation antihistamines. These antihistamines have a lower sedative effect than first-generation antihistamines.
Ques: Are antihistamines anti-inflammatory? (2 marks)
Ans:Antihistamines have recently been discovered to have anti-inflammatory effects that go beyond simply inhibiting histamine receptors. According to new research, these medications reduce the expression of cell adhesion molecules.
Ques: How quickly does an antihistamine take effect? (2 marks)
Ans: Antihistamine medications typically begin working within 30 minutes of administration and are most effective within 1-2 hours. Antihistamines are more effective when used on a regular basis as a preventative measure rather than just when symptoms appear.
Ques: Is it true that drinking water lowers histamine levels? (2 marks)
Ans: Water helps with histamine clearance because it removes more than 95 percent of excess histamines from the body. Drinking plenty of water keeps histamine levels stable and healthy, allowing your body to digest them effectively.
Ques: How do antihistamines keep allergies at bay? (2 marks)
Ans: Antihistamines inhibit the action of histamines, which are released when our bodies come into contact with substances such as pollen, dust mites, and so on. These components cause histamines to be released, causing allergies such as nasal swelling, running nose and eyes, and so on. Antihistamines prevent allergies by blocking the action of histamines.
Ques: What is the cause of an allergy? (3 marks)
Ans: Allergies occur when your immune system reacts to a foreign substance, such as pollen or bee venom. Allergies are the reactions of your body when it comes into contact with foreign substances. In this case immune system of the body produces various types of antibodies. When our body comes in contact with the allergen, a burning sensation is sensed on our skin, sinuses, airways and digestive system.
Ques: What does the first generation compounds do not cross the blood-brain barrier as easily as Second Generation compounds? (3 marks)
Ans: The reason why first generation compounds do not cross the blood-brain barrier as easily as second generation compounds are as follows:
- Lipophobicity
- Large molecular weight
- Charged by electrostatic attraction
Ques: What are the substance that will cause allergies? (3 marks)
Ans: The substances that can cause allergies are as follows:
- Dust
- Saliva
- Urine
- Food.
- Pet Dander
- Mold.
- Insect Stings
- Insect bites and stings.
- Latex.
- Medications and drugs
Ques: What are the types of antihistamines? (4 marks)
Ans: The types of antihistamines are as follows:
- H1 Antihistamines: It is a type of antihistamines that inhibit the activity of the H1 receptor. The most common side effect of H1 antihistamines include sedation. H1 antihistamines can work as inverse agonist or neural receptor agnonists.
- H2 Antihistamines: H2 Antihistamine inhibit the activity of the H2 receptor which are most commonly found in the gastric mucosa parietal cells. It will induce the secretion of acid and help reduce gastroesophageal reflux disorder.
- H3 Antihistamines: H3 Antihistamine is a type of antihistamine that inhibit the activity of H3 receptor which are found in the brain. It will also inhibit the autoreceptors located on histaminergic nerve terminals, which modulate the release of histamine.
- H4 Antihistamines: H4 Antihistamine is a type of antihistamine that inhibit the activity of H4 receptor. Example of H4 Antihistamine includes Thioperamide and JNJ 7777120
Ques: What are the side effects of antihistamine? (3 marks)
Ans: Some of the common side effects of antihistamine are as follows:
- Drowsiness.
- Dry mouth, dry eyes.
- Dizziness and headache.
- Mucous thickening in the airways.
- Headache.
- Cough.
- Tiredness.
- Sore throat.
- Nausea or vomiting.
Ques: In how many categories antihistamine are classified? (2 marks)
Ans: Antihistamine are classified into two categories which are as follows:
- First Generation Antihistamine (H1 Antihistamines): H1 Antihistamines is a type of function that is formed by binding of H1 receptors of histamines in the smooth cell, mast cell and endothelial cells in the body. It is also connected with the tuberomammillary nucleus found inside the brain.
- Second Generation of Antihistamine (H2 Antihistamines): In general, they do not cause sedation and drying those first-generation antihistamines do. They inhibit the activity of the H2 receptor which are most commonly found in the gastric mucosa parietal cells. It will induce the secretion of acid and help reduce gastroesophageal reflux disorder.
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