[Chemistry Class Notes] on Antihistamine Pdf for Exam

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.

 

 

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.