Histamine and Antihistamines

Histamine and Antihistamines

1. What is Histamine?

Histamine is a chemical (amine) released by the body as a response to inflammatory reactions, allergies, digestive stimulation, and nerve signal transmission.

2. Histamine Formation Process

Histamine is produced from histidine through the decarboxylation process.

3. Histamine Storage in the Body

• Mast cells: These are the primary storage sites for histamine, in an inactive complex form.

• Tissues: Contain fewer mast cells, concentrated in the skin, respiratory mucosa, and digestive system.

4. Histamine Reserves

Mast cells (basophils) are the main storage sites for histamine.

5. Causes of Histamine Release

Histamine is released due to various factors such as:

• Immune response (antigen-antibody complex)

• Irritants like sunlight, venom, morphine

6. Histamine Receptors

Histamine acts through four main receptor types: H1, H2, H3, and H4.

7. Histamine Effects:

• H1: Distributed in smooth muscle, endothelial cells, causing bronchoconstriction, vasodilation, and increased vascular permeability.

• H2: Distributed in gastric cells, causing stimulation of gastric acid secretion.

8. Effects of Histamine Stimulation on Receptors:

• H1: Causes bronchospasm, intestinal smooth muscle contraction, increased vascular permeability, and stimulation of sensory nerves.

• H2: Increases the risk of peptic ulcers.

9. Cardiovascular and Skin Effects:

H1 and H2 stimulation can lead to urticaria (hives) and hypotension (low blood pressure).

10. Antihistamine H1 Antagonists

• Antihistamine H1 antagonists inhibit the biological effects of histamine.

• First-generation antihistamine H1 can cross the blood-brain barrier.

• Antihistamine H1 exists in two generations: first generation and second generation.

11. Applications of Antihistamines:

• Allergies: Antihistamine H1 is used to treat allergic diseases.

• Peptic ulcers: Antihistamine H2 is used to treat peptic ulcers.

12. Classification of Antihistamines:

• Antihistamines: Antihistamine H1.

• H2 Blockers: Antihistamine H2.

13. First-generation Antihistamine H1:

• Has anticholinergic, anti-alpha-adrenergic, and anti-serotonin effects.

• Peripheral effects.

• Short half-life.

• Examples: Clophenylramine, Diphenhydramine, Promethazine.

14. Second-generation Antihistamine H1:

• Peripheral effects.

• Does not cause sedation, anticholinergic, or antiemetic effects.

• Examples: Loratadine, Fexofenadine.

15. Mechanism of Action of Antihistamine H1:

• Competitively inhibits histamine at the H1 receptor.

• Does not affect the synthesis or release of histamine.

16. Pharmacological Effects of First-generation Antihistamine H1:

• Anticholinergic: Dry mouth, urinary retention, blurred vision (Diphenhydramine, Promethazine).

• Anti-alpha-adrenergic: Hypotension, tachycardia (Promethazine).

• Anti-serotonin: Increased appetite (Cyproheptadine).

17. Other Applications of Antihistamine H1:

• Motion sickness: Diphenhydramine, Dimenhydrinate.

• Reducing nausea and vomiting in pregnant women: Doxylamine, Pyridoxine.

• Reducing vertigo due to vestibular disorders: Meclizine.

• Reducing extrapyramidal symptoms: Diphenhydramine.

18. Side Effects of Antihistamine H1:

• First generation: Allergic skin reactions, cross-sensitivity.

• Second generation: Astemizole or terfenadine can cause heart rhythm disturbances (not used).

19. Indications for Antihistamine H1:

• Allergic rhinitis, urticaria, allergic conjunctivitis, atopic dermatitis.

20. Indications for First-generation Antihistamine H1:

• Motion sickness.

• Pregnancy.

• Vertigo due to vestibular disorders.

• Limited treatment of insomnia.

21. Contraindications for First-generation Antihistamine H1:

• Glaucoma, gastrointestinal obstruction, urinary obstruction.

• Drivers, machinery operators (do not use medications containing piperazine).

22. Dual-acting Antihistamine H1:

• Stabilize mast cells and have antagonist effects.

Note: This article is for general information purposes only and does not substitute for professional medical advice. You should consult with a healthcare professional before using any medication.