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H-1 ANTAGONISTS

  • H1 antagonists, also known as H1 blockers or antihistamines, are a class of drugs that inhibit the action of histamine at the H1 receptor sites.

  • They are widely used to treat various allergic conditions and are classified into three main categories:

1. First-Generation H-1 Antagonists

2.Second-Generation H-1 Antagonists

3. Mast Cell Stabilizers

Medical Uses of H-1 Antagonists

  • Allergic Rhinitis: Reduces symptoms like sneezing and nasal congestion.

  • Allergic Conjunctivitis: Alleviates itching and redness in the eyes.

  • Urticaria: Treats hives and skin rashes.

  • Angioedema: Reduces swelling beneath the skin.

  • Pruritus: Relieves itching from atopic dermatitis and insect bites.

  • Nausea and Vomiting: Some antihistamines have antiemetic properties.

  • Sedation: First-generation antihistamines are used as sleep aids due to their sedative effects.

Side Effects

  • Sedation and Drowsiness (especially with first-generation agents)

  • Blurred Vision

  • Dry Mouth

  • Constipation or Diarrhea

  • Nausea and Vomiting

  • Increased Appetite and Weight Gain

  • Anxiety or Insomnia

  • Dry Cough

Classification

classification of H-1 ANTAGONISTS mast cell stabilizers
  1. First-Generation H1 Antagonists

Characteristics:

i. Sedative Effects: Easily cross the blood-brain barrier, causing drowsiness.

ii. Anticholinergic Activity: Block muscarinic acetylcholine receptors, leading to side effects like dry mouth and blurred vision.

iii. Affordable and Widely Available: Oldest class of antihistamines.

Mechanism of Action:

i. Histamine Blockade: Competitively inhibit histamine binding at H1 receptors.

ii. G-Protein Coupled Receptor Inhibition: Prevent activation of phospholipase C, reducing inositol phosphate (IP₃) and diacylglycerol (DAG) formation.

iii. Reduced Intracellular Calcium: Decreases the release of inflammatory mediators.

Functions:

i. Decrease Vascular Permeability: Reduces edema and swelling.

ii. Alleviate Pain and Headache: Minimizes discomfort associated with allergies.

iii. Prevent Hypotension: Stabilizes blood pressure by inhibiting vasodilation

  1. Second-Generation H1 Antagonists

Characteristics:

i. Non-Sedative: Less likely to cross the blood-brain barrier, resulting in minimal sedation.

ii. Selective Action: More specific to peripheral H1 receptors.

iii. Hydrophilic Nature: Limits central nervous system penetration.

Mechanism of Action:

Similar to first-generation antihistamines but without significant central nervous system effects.

Functions:

i. Alleviate Allergy Symptoms: Effective in treating allergic rhinitis, conjunctivitis, and urticaria without causing drowsiness.

  1. Mast Cell Stabilizers

Characteristics:

i. Prevent Histamine Release: Stabilize mast cell membranes, inhibiting degranulation.

ii. Anti-Inflammatory Activity: Reduce the release of other inflammatory mediators.

Mechanism of Action:

i Calcium Channel Blockade: Inhibit IgE-mediated calcium influx into mast cells.

ii. Prevent Degranulation: Without intracellular calcium, histamine vesicles cannot fuse with the cell membrane.


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