Chapter 46: Allergies & Allergic Reactions – Drug Management

The chapter, "Allergies and Allergic Reactions," provides a comprehensive overview of hypersensitivity responses and their management, focusing particularly on allergic rhinitis and life-threatening anaphylaxis. Allergies are defined as exaggerated immune system reactions where soluble protein molecules called antibodies (immunoglobulins or Ig), typically IgE, bind specifically to foreign substances known as antigens or allergens, triggering inflammation and tissue damage. The four primary types of hypersensitivity reactions are detailed under the Coombs and Gell classification: Type 1 (immediate, IgE-mediated, involving mast cell degranulation and histamine release, exemplified by asthma and anaphylaxis); Type 2 (IgG-mediated cytotoxic action, like transfusion reactions); Type 3 (immune complex deposition, seen in autoimmune diseases); and Type 4 (delayed, cell-mediated reactions, such as contact dermatitis). For the most severe Type 1 reaction, anaphylaxis, immediate administration of intramuscular epinephrine is vital to counteract shock and airway obstruction, often supplemented by injectable antihistamines and rapid intravenous fluids. For managing chronic conditions like allergic rhinitis (seasonal or perennial), which involves IgE triggering mediator release in the nasal mucosa, a stepwise pharmacotherapeutic approach is recommended, starting with avoidance measures. Intranasal corticosteroids are generally considered the most potent first-line agents, working to reduce inflammation by inhibiting various immune cells and mediators, although maximal effect may take up to two weeks. Second-line treatments include antihistamines (H1 blockers), categorized as first-generation (sedating, high anticholinergic risk, e.g., diphenhydramine) or second-generation Non-Sedating Antihistamines (NSAs) (preferred, e.g., loratadine, fexofenadine), often paired with nasal decongestants like pseudoephedrine for congestion relief. Decongestants constrict nasal blood vessels but carry risks of systemic cardiovascular stimulation or, if used topically for more than a few days, rebound congestion known as rhinitis medicamentosa. Prophylactic measures include allergen avoidance, specific immunotherapy (desensitization), and the use of mast cell stabilizers like intranasal cromolyn. Successful management requires careful consideration of special populations (pediatric, geriatric, pregnant women) and diligently monitoring patient adherence and outcomes.