This document discusses antihistamines, which are pharmaceutical agents that inhibit the actions of histamine. It describes how histamine is produced and its effects on different receptor types (H1-H4). The main types of antihistamines are H1 receptor blockers, which provide relief from allergic symptoms, and H2 receptor antagonists, which inhibit gastric acid secretion. The document also briefly mentions other classes of antihistamines that target H3/H4 receptors or inhibit histamine production.

1. Antihistaminics

2. Histamine
• physiologically active amine C5H9N3
• plant and animal tissue
• Released from mast cells as part of an allergic
reaction (IgE / PG / LT / TNF-A / SP/ NKA
Complement-mediated)
-Gastric Secretion
-Capillary Dilatation
-Bronchoconstriction
-Hypotension Imidazoline
ring and amine gp

3. Synthesis
• Decarboxylation of AA Histidine by Histidine
decarboxylase
• Mast cells and basophils

4. Mechanism of Action
• Combining with specific cellular receptors on
cells viz H1-H4 ; Leucocyte diapedesis 
Neutrophil Chemotaxis  Mast Cell
Degranulation
• Primarily H1-mediated in Lewis’ Triple
Response / UV
• H2-H4 blockers / Atypical Antihistaminics
found largely ineffective

5. Type Location Function
H1 Found on smooth muscles,
endothelium, and central
nervous system tissue
Causes vasodilation, bronchoconstriction
bronchial smooth muscle contraction, separation
of endothelial cells (responsible for hives),
and pain and itching due to insect stings; the
primary receptors involved in allergic rhinitis
symptoms and motion sickness; sleep regulation.
H2 Located on parietal cells Primarily stimulate gastric acid secretion
And we use antiacidic drugs for this receptor
H3 Found on central nervous
system and to a lesser
extent peripheral nervous
system tissue
Decreased neurotransmitter release:
histamine, acetylcholine, norepinephrine,
serotonin
H4 Found primarily in
the basophils and in
the bone marrow. It is also
found onthymus, small
intestine, spleen, andcolon.
Plays a role in chemotaxis.

6. 1st
Antihistaminic
• Piperoxan
• Discovered in 1933 by Jeff Forneau and Daniel
Bovent while developing a guinea pig animal
model of anaphylaxis
• They received the Nobel Prize in 1957

7. Antihistaminics
• Pharmaceutical agents that inhibit the action of
histamine
• blocking its attachment to histamine receptors
(Classic Antihistaminics)
• inhibiting the enzymatic activity of
histidine decarboxylase (atypical antihistaminics)
• The term antihistamine only refers to H1 receptor
antagonists

8. Classification
H1 Blockers
Traditional Sedatives (esrt 1st
Gen <1981)
Low-Sedation agents (erst 2nd
Gen)
H2 / H3 / H4-receptor Antagonists
Tricyclic Antihistaminics
Histidine Decarboxylase Enzyme-Inhibitors
Mast Cell Degranulation Inhibitors

9. • a

10. Traditional Sedative H1 blockers
• a

11. Low-Sedation H1 Blockers
• aa

12. H2 Receptor Antagonists
• Parietal Cells – Inhibit Gastric Secretion
• Cimetidine
• Ranitidine
• Nizatidine
• Roxatidine
• Famotidine
• Lafutidine

13. H3 Receptor Antagonists
• Nootropics / Neuromodulators
• Ciproxifan
• Clobenpropit
• Conessine

14. H4 Receptor Antagonists
• Immunomodulators
• Thioperamide
• VUF-6002
• JNJ 7777120

15. Histidine Decarboxylase Inhibitors
• Tritoqualine
• Catechin

16. Mast Cell Degranulation Inhibitors
• Sodium Cromogylcate
• Nedocromil
• Inhalational Agents ; Ineffective in Cutaneous
Dermatoses

17. THANK YOU