H-1 antagonists

Discover how H-1 antagonists work to treat allergies by blocking histamine receptors and explore their types and common uses.

• 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 H1 Antagonists
2. Second-Generation H1 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

1. First-Generation H1 Antagonists

Characteristics:

• • Sedative Effects: Easily cross the blood-brain barrier, causing drowsiness.
  • Anticholinergic Activity: Block muscarinic acetylcholine receptors, leading to side effects like dry mouth and blurred vision.
  • Affordable and Widely Available: Oldest class of antihistamines.

Mechanism of Action:

• • Histamine Blockade: Competitively inhibit histamine binding at H1 receptors.
  • G-Protein Coupled Receptor Inhibition: Prevent activation of phospholipase C, reducing inositol phosphate (IP₃) and diacylglycerol (DAG) formation.
  • Reduced Intracellular Calcium: Decreases the release of inflammatory mediators.

Functions:

• • Decrease Vascular Permeability: Reduces edema and swelling.
  • Alleviate Pain and Headache: Minimizes discomfort associated with allergies.
  • Prevent Hypotension: Stabilizes blood pressure by inhibiting vasodilation.

2. Second-Generation H1 Antagonists

Characteristics:

• • Non-Sedative: Less likely to cross the blood-brain barrier, resulting in minimal sedation.
  • Selective Action: More specific to peripheral H1 receptors.
  • Hydrophilic Nature: Limits central nervous system penetration.

Mechanism of Action:

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

Functions:

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

3. Mast Cell Stabilizers

Characteristics:

• • Prevent Histamine Release: Stabilize mast cell membranes, inhibiting degranulation.
  • Anti-Inflammatory Activity: Reduce the release of other inflammatory mediators.

Mechanism of Action:

• • Calcium Channel Blockade: Inhibit IgE-mediated calcium influx into mast cells.
  • Prevent Degranulation: Without intracellular calcium, histamine vesicles cannot fuse with the cell membrane.

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