Antihistamines for allergies

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Antihistamines for allergies

Prerequisite basic sciences

Prerequisite basic sciences

Prerequisite basic sciences


Antihistamines for allergies


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Antihistamines for allergies

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USMLE® Step 1 style questions USMLE

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A 35-year-old taxi driver presents to the office because of itchy, watery eyes and excessive sneezing that have become more severe with the change of seasons. The patient has taken an over-the-counter medication in the past with moderate relief of the symptoms, but it made the patient very drowsy. Past medical history is significant for eczema. Family history is noncontributory. Vitals are within normal limits. Physical examination shows a prominent nasal crease. The nasal turbinates are boggy and bluish-gray. There is copious watery nasal mucus. The patient asks the physician to switch to a medication that will not impair his ability to drive. Which of the following medications is most appropriate for this patient?  

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Cetirizine p. 710


Antihistamines for allergies, also known as H1 blockers, are medications that are primarily used to treat allergic reactions, such as urticaria, angioedema, and allergic rhinitis.

These conditions are related to an increased release of histamine.

Now, H1 blockers work by blocking the effects of histamine in tissues that have H1 receptors, thereby alleviating symptoms of allergic reactions.

In order to understand how antihistamines work, first we need to talk briefly about histamine and allergic reactions.

Histamine is a small molecule that’s mainly produced by mast cells and basophils.

Once released, they cause local inflammation and vasodilation. However, they are also present in the brain as neurotransmitters, and they are produced by enterochromaffin cells in the stomach to increase gastric acid secretion.

Okay, so in order to develop an allergic reaction, an allergen, say pollen, needs to enter the body and cause the activation of B cells.

Activated B-cells produce IgE antibodies that get released into the bloodstream and bind to mast cells.

The mast cells are now “primed,” meaning that if pollen enters the body again in the future, the mast cells degranulate and release their histamine into the local tissue.


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  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  4. "Antihistamines for Allergic Rhinitis Treatment from the Viewpoint of Nonsedative Properties" International Journal of Molecular Sciences (2019)
  5. "The Role of Histamine in the Pathophysiology of Asthma and the Clinical Efficacy of Antihistamines in Asthma Therapy" International Journal of Molecular Sciences (2019)
  6. "Antihistamines and allergy" Australian Prescriber (2018)

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