🫀Hypersensitivity: overview and type I

Question 1

What are hypersensitivity reactions?

Answer 1

Hypersensitivity = exaggerated immune response to harmless Ags → tissue damage/inflammation.

Question 2

What are the four types of hypersensitivity?

Answer 2

• Type I: IgE-mediated (allergy, anaphylaxis)
• Type II: Ab-mediated (cytotoxic, e.g., Rh disease)
• Type III: Immune complex-mediated (e.g., SLE, post-strep GN)
• Type IV: T-cell mediated (delayed, e.g., MS, T1D)

Question 3

What is the mechanism of type I hypersensitivity?

Answer 3

• IgE binds mast cells → Sensitization
• Re-exposure → Allergen cross-links IgE → Mast cell degranulation → Histamine, cytokines released → Inflammation, bronchoconstriction, allergic symptoms

Question 4

What is the role of IgE in type I hypersensitivity?

Answer 4

• IgE binds mast cells/basophils, primes for future exposure.
• Allergen cross-links IgE → degranulation → histamine, cytokines → allergic rxn.

Question 5

How do mast cells respond to allergens?

Answer 5

• Mast cells bind IgE → primed for allergen exposure.
• Allergen cross-links IgE → degranulation → release of histamine, cytokines, leukotrienes → allergic response.

Question 6

What are the clinical features of allergic rhinitis and asthma?

Answer 6

• Allergic rhinitis: Sneezing, rhinorrhea, nasal congestion, itchy eyes/nose, post-nasal drip.
• Asthma: Wheezing, SOB, chest tightness, coughing, especially at night/morning.

Question 7

What is anaphylaxis and how is it triggered?

Answer 7

• Anaphylaxis: Severe, rapid allergic reaction.
• Triggers: Allergen exposure (e.g., food, insect stings, meds).
• Mechanism: IgE-mediated mast cell/basophil degranulation → release of histamine, cytokines.

Question 8

What is the late-phase reaction in allergy?

Answer 8

• Late-phase reaction: Occurs 4-6 hours after initial exposure.
• Mechanism: Infiltration of eosinophils, neutrophils, and T-cells.
• Outcome: Persistent inflammation, tissue damage, and prolonged symptoms.

Question 9

How does the Th2 response contribute to allergy?

Answer 9

• Th2 activation: Stimulates B-cells to produce IgE antibodies.
• IL-4, IL-5, IL-13: Key cytokines that promote IgE production and eosinophil recruitment.
• Outcome: IgE-mediated mast cell degranulation, inflammation, and allergic symptoms.

Question 10

What are examples of common allergens?

Answer 10

• Pollen: Tree, grass, and weed pollens
• Dust mites: Proteins in dust mite feces
• Mold: Fungal spores
• Animal dander: Proteins in pet skin flakes, saliva, and urine
• Food allergens: Peanuts, shellfish, eggs, milk, wheat
• Insect stings: Bee, wasp, hornet venom