Histamines and Antihistamines Quiz

Histamine and its Effects

• Histamine is formed by the enzyme histidine decarboxylase.
• Histamine binding to H1 receptors causes smooth muscle contraction of the gastrointestinal (GI) tract.
• The main effect of histamine binding to H2 receptors is gastric acid secretion.
• Mast cells contain the highest number of immune cells for immune function.

Histamine Receptors

• Histamine binding to H1 receptors on endothelial cells causes increased permeability and edema.
• Stimulation of H3 auto receptors by histamine causes inhibition of histamine release.
• The triple response to histamine is characterized by redness, wheal, and flare reactions.
• Histamine binding to H1 receptors in the airway smooth muscle causes bronchoconstriction.

Antihistamines

• Sedating antihistamines can enter the brain and cause sedation, and are not recommended to be taken with other depressants or alcohol.
• Non-sedating antihistamines, such as loratadine, are considered cardiac safe and do not cause sedation.
• The primary mechanism of action of non-sedating antihistamines is competitive inhibition of histamine binding to H1 receptors.

Allergic Reactions

• Type I hypersensitivity reactions involve the formation of IgE antibodies and sensitization of tolerance pathways.
• The specific allergic reaction that involves the activation of Th2-driven pathways and the formation of IgE antibodies is anaphylaxis.

Pharmacology

• Tertiary antihistamines, such as terfenadine, are known to raise the QT interval and are not considered cardiac safe.
• The H1 receptor is primarily responsible for mediating the itch sensation.
• Sedating antihistamines have a duration of action of 4-6 hours.
• Diphenhydramine is used in the treatment of both insomnia and as an enhancer for the effects of analgesics.
• Cetirizine is known for its limited crossing of the blood-brain barrier and high affinity for P-glycoprotein.
• Chlorpheniramine is considered a 'dirty drug' with many side effects, including antimuscarinic effects and contraindications with sedating agents and CNS depressants.