Allergy

Educational clinical summary of Allergy for doctors and students: definition, workup, management, and key caveats. Not a substitute for guidelines.

Maintained by Dr. Vivek Karn Review process: editorial policy

Disclaimer: This document is intended for informational and educational purposes only and does not constitute medical advice. It is designed as a clinical reference for healthcare professionals and should not be used as a substitute for professional medical judgment. Patient care decisions must always be made in consultation with a qualified healthcare provider, considering the individual patient's clinical circumstances, local guidelines, and the most current evidence. The information presented here may not encompass all possible scenarios, treatments, or side effects. Always verify dosages, indications, and contraindications with official drug prescribing information.

Allergy

Overview

Definition: An allergy is a hypersensitivity reaction initiated by immunological mechanisms. These reactions are typically specific and reproducible upon exposure to a given antigen (allergen). They are broadly classified by the Gell and Coombs system into four types, though Type I (IgE-mediated) reactions are most commonly referred to as "allergies" in clinical practice.
Epidemiology: Allergic diseases are among the most common chronic diseases globally, affecting a significant portion of the population.
- Prevalence: Estimates vary, but allergic rhinitis affects 10-30% of adults and up to 40% of children. Asthma affects around 10-12% of the population. Food allergies affect approximately 8% of children and 2-4% of adults. Eczema affects 15-20% of children and 1-3% of adults.
- Demographics: Allergic diseases often begin in childhood, with a substantial number persisting into adulthood. There is an increasing prevalence observed over recent decades, particularly in industrialized nations, contributing to the "allergic march" (the natural progression from atopic dermatitis to food allergy, allergic rhinitis, and asthma).
Classification:
- Type I (Immediate Hypersensitivity - IgE-mediated): Most common form of allergy. Involves IgE antibodies binding to mast cells and basophils, leading to mediator release (e.g., histamine) upon re-exposure to allergen. Examples: anaphylaxis, allergic rhinitis, allergic asthma, food allergy, acute urticaria/angioedema.
- Type II (Cytotoxic Hypersensitivity): Involves IgG or IgM antibodies binding to cell surface antigens, leading to cell lysis. Examples: drug-induced hemolytic anemia, ABO incompatibility. (Less commonly referred to as "allergy" in the typical sense).
- Type III (Immune Complex-mediated Hypersensitivity): Involves immune complex deposition (antigen-antibody complexes), leading to inflammation. Examples: serum sickness, Arthus reaction, some vasculitides. (Less commonly referred to as "allergy").
- Type IV (Delayed-Type Hypersensitivity - Cell-mediated): Involves T-cell mediated reactions, typically manifesting 24-72 hours after exposure. Examples: contact dermatitis (e.g., poison ivy, nickel), tuberculin skin test, drug-induced hypersensitivity syndrome (DRESS).
- Non-IgE-mediated Food Allergy: Reactions not involving IgE, often gut-mediated. Examples: food protein-induced enterocolitis syndrome (FPIES), food protein-induced proctocolitis (FPIAP), celiac disease (though an autoimmune disease, it has an immune-mediated food trigger).
- Mixed IgE and Non-IgE-mediated Reactions: Examples: eosinophilic esophagitis (EoE), atopic dermatitis.

Pathophysiology

Mechanism:
- IgE-mediated Allergy (Type I):
  1. Sensitization: Initial exposure to an allergen leads to activation of T-helper 2 (Th2) lymphocytes. Th2 cells stimulate B lymphocytes to produce allergen-specific IgE antibodies.
  2. IgE Binding: These IgE antibodies bind to high-affinity FcεRI receptors on the surface of mast cells (in tissues) and basophils (in circulation).
  3. Effector Phase: Upon re-exposure, the allergen cross-links two adjacent IgE molecules on the mast cell/basophil surface, triggering degranulation.
  4. Mediator Release: Degranulation releases preformed mediators (e.g., histamine, tryptase, heparin, proteases) and leads to synthesis of newly formed mediators (e.g., leukotrienes, prostaglandins, platelet-activating factor, cytokines).
  5. Clinical Manifestations: These mediators cause smooth muscle contraction (bronchospasm), increased vascular permeability (edema, hypotension), vasodilation (flushing), glandular secretions (rhinorrhea, watery eyes), and nerve stimulation (pruritus, pain).
- Non-IgE-mediated (e.g., Type IV): Involves allergen presentation by antigen-presenting cells to T lymphocytes. Sensitized T cells proliferate and, upon re-exposure, release cytokines that recruit and activate inflammatory cells (e.g., macrophages), leading to tissue damage.
Risk Factors:
- Genetic Predisposition (Atopy): Strongest risk factor. A familial history of allergic diseases (e.g., asthma, allergic rhinitis, eczema, food allergy) significantly increases an individual's risk. Specific gene loci associated with allergy (e.g., those encoding filaggrin, components of the immune system).
- Environmental Exposures:
  - Allergens: Exposure to indoor (dust mites, pet dander, mold, cockroach) and outdoor (pollen) allergens.
  - Pollution: Air pollutants (e.g., particulate matter, ozone, nitrogen dioxide) exacerbate allergic inflammation.
  - Diet: Certain dietary patterns, though evidence for specific links remains debated (e.g., Western diet).
  - Microbiome: Alterations in gut and airway microbiota (dysbiosis) may influence immune development.
- Early Life Events: Mode of delivery (C-section), early antibiotic use, lack of microbial exposure.
- "Hygiene Hypothesis": Reduced exposure to microbes and infections in early life may lead to an imbalance in immune system development, favoring a Th2-dominant response.
- Other: Obesity, smoking (active and passive), viral infections (especially respiratory syncytial virus in infancy).
Protective Factors:
- Biodiversity/Early Microbial Exposure: Living on a farm, exposure to a diverse microbiome in early life.
- Vaginal Delivery and Breastfeeding: May influence gut microbiome development.
- Early Allergen Introduction (for Food Allergy): For high-risk infants, early introduction of common allergenic foods (e.g., peanut, egg) from 4-6 months of age can reduce the risk of developing food allergies.
- Omega-3 Fatty Acids: Some evidence suggests a potential role in modulating immune responses, though not consistently proven as a primary protective factor.

Clinical Presentation

Signs and Symptoms

Clinical presentation varies widely depending on the allergen, route of exposure, and target organ system. - Cardinal Features: * Anaphylaxis: Rapid onset of life-threatening symptoms involving two or more organ systems (skin/mucosa, respiratory, cardiovascular, gastrointestinal). Often includes pruritus, urticaria, angioedema, flushing, dyspnea, wheezing, stridor, hypotension, dizziness, syncope, abdominal pain, vomiting, diarrhea. * Allergic Rhinitis: Sneezing, rhinorrhea (clear, watery), nasal congestion, nasal pruritus, ocular pruritus, tearing, post-nasal drip, cough. * Allergic Asthma: Cough, wheezing, shortness of breath, chest tightness, worse at night or with exercise/allergen exposure. * Food Allergy: Oral pruritus/tingling, urticaria, angioedema (lips, face, throat), vomiting, diarrhea, abdominal pain, respiratory symptoms (wheezing, dyspnea), cardiovascular symptoms (hypotension). * Atopic Dermatitis (Eczema): Pruritus, erythematous patches, papules, vesicles, lichenification, dry skin. Often symmetrically distributed in flexural areas (antecubital, popliteal fossae), neck, wrists, ankles. * Acute Urticaria/Angioedema: Pruritic, transient, erythematous, edematous wheals (urticaria); deeper tissue swelling (angioedema), often involving face, lips, tongue, larynx, extremities, genitals. * Contact Dermatitis: Pruritus, erythema, papules, vesicles, bullae, weeping, crusting, scaling, usually localized to area of contact. - Early Signs: * Anaphylaxis: Pruritus, flushing, feeling of warmth, tingling around the mouth, mild urticaria, feeling of impending doom. * Respiratory: Nasal pruritus, sneezing (rhinitis); mild cough (asthma). * Gastrointestinal: Oral pruritus (food allergy). - Advanced Signs: * Anaphylaxis: Severe dyspnea, stridor, cyanosis, profound hypotension, syncope, incontinence, cardiovascular collapse, loss of consciousness. * Respiratory: Severe wheezing, silent chest, respiratory distress, accessory muscle use (asthma). * Skin: Widespread urticaria, extensive angioedema. - Atypical Presentations: * Food Allergy: Exercise-induced anaphylaxis (often food-dependent), oral allergy syndrome (pollen-food syndrome), food protein-induced enterocolitis syndrome (FPIES) with repetitive vomiting, pallor, lethargy, sometimes progressing to shock, typically without IgE-mediated skin/respiratory symptoms. * Drug Allergy: Drug reaction with eosinophilia and systemic symptoms (DRESS syndrome) - fever, rash, lymphadenopathy, multiorgan involvement, eosinophilia, atypical lymphocytes, delayed onset (2-8 weeks). Serum sickness-like reactions. Fixed drug eruptions. * Insect Sting Allergy: Large local reactions (swelling >10 cm, lasting >24 hours) without systemic symptoms.

Physical Examination

Findings are highly dependent on the affected organ system and severity. - Inspection: * Skin: Urticarial wheals (erythematous, edematous, evanescent), angioedema (puffy swelling, non-pruritic, non-pitting), erythematous patches/papules/vesicles/crusting (eczema, contact dermatitis), flushing, pallor (anaphylaxis). * Respiratory: Nasal crease, allergic shiners (periorbital edema/darkness), mouth breathing (rhinitis); accessory muscle use, retractions, pursed-lip breathing, cyanosis (asthma/anaphylaxis). * General: Swelling of face/lips/tongue/uvula, signs of respiratory distress, altered mental status. - Palpation: * Skin: Edema (urticaria, angioedema). * Lymph Nodes: Lymphadenopathy (DRESS syndrome, severe infections). * Pulse: Tachycardia, weak/thready pulse (anaphylaxis). - Special Tests: * Peak Expiratory Flow (PEF) / Spirometry: To assess airflow obstruction in asthma. Significant reduction in PEF in acute exacerbations. * Oximetry: To assess oxygen saturation in respiratory distress. * Blood Pressure: Hypotension is a critical sign of anaphylaxis.

Diagnostic Approach

History Taking

Crucial for identifying triggers and determining the type of reaction. - Key Questions: * Symptoms: Detailed description of symptoms (type, severity, onset, duration, progression). * Timing: Relation of symptoms to exposure (minutes for IgE-mediated, hours-days for delayed). * Exposure: Specific allergen suspected (food, drug, insect sting, environmental exposure). Amount of exposure. * Reproducibility: Has this happened before? Under what circumstances? * Route of Exposure: Ingestion, inhalation, injection, skin contact. * Co-factors: Exercise, NSAIDs, alcohol, illness, stress (especially for food-dependent exercise-induced anaphylaxis or chronic urticaria). * Management of Previous Reactions: What was done? Was epinephrine used? Did it help? * Medical History: Personal history of atopic diseases (eczema, asthma, rhinitis, other allergies). * Family History: Allergies in first-degree relatives. * Medications: All current and recent medications, including over-the-counter and herbal supplements (crucial for drug allergy). * Environmental: Home environment, pets, hobbies, occupational exposures. - Red Flags: * Rapid onset of symptoms after exposure. * Involvement of multiple organ systems (especially skin/mucosa + respiratory OR skin/mucosa + cardiovascular). * Respiratory distress (dyspnea, wheezing, stridor, accessory muscle use). * Signs of shock (hypotension, pallor, dizziness, syncope, altered mental status). * Recurrent, unexplained anaphylaxis.

Laboratory Tests

Used to confirm sensitization, but not necessarily clinical allergy. - First-Line: * Allergen-Specific IgE (sIgE) Blood Test (ImmunoCAP, RAST): Measures levels of IgE antibodies directed against specific allergens in the blood. Less sensitive than skin prick testing but useful if skin testing is contraindicated (e.g., severe eczema, dermographism, inability to stop antihistamines, risk of severe reaction). Results reported in kU/L. High levels indicate sensitization but do not always predict clinical reactivity. * Skin Prick Testing (SPT): Gold standard for diagnosing IgE-mediated allergies. Small amounts of allergen extract are applied to the skin and then pricked. A positive reaction (wheal and flare) within 15-20 minutes indicates sensitization. Read with positive (histamine) and negative (saline) controls. Requires discontinuation of antihistamines (typically 3-7 days prior). - Confirmatory: * Oral Food Challenge (OFC): Gold standard for diagnosing food allergy. Performed under medical supervision in a controlled environment, where gradually increasing amounts of the suspected food are ingested. Used to confirm diagnosis or assess resolution of allergy. High risk, requires emergency equipment. * Drug Challenge: For drug allergy, similar to OFC but with drugs. Only performed when risk is low or alternative drugs are limited. * Patch Testing: For Type IV (delayed) hypersensitivity, particularly contact dermatitis. Allergens are applied to the skin under occlusive patches and read at 48 and 72-96 hours. * Basophil Activation Test (BAT): Flow cytometry-based test measuring upregulation of activation markers (e.g., CD63, CD203c) on basophils after stimulation with allergen. Can be useful in some drug allergies (e.g., beta-lactam) and chronic urticaria. Not widely available or standardized. * Component Resolved Diagnostics (CRD): Measures IgE to specific protein components within an allergen extract (e.g., Ara h 2 for peanut, Bet v 1 for birch pollen). Improves diagnostic precision by distinguishing true sensitization from cross-reactivity and can predict reaction severity or persistence. - Monitoring: * Serum Tryptase: A mast cell-specific enzyme. Elevated levels can confirm mast cell degranulation, supporting a diagnosis of anaphylaxis. Peak levels typically occur 1-2 hours after onset of symptoms and normalize within 6-12 hours. Basal tryptase may be elevated in mastocytosis. * Specific IgE Levels: Can be monitored over time, especially in food allergy, to assess trends and potential for allergy resolution.

Imaging

Generally not primary for allergy diagnosis, but may be used to assess complications or differentiate from other conditions. - Recommended: * Chest X-ray: In severe asthma exacerbation or suspected pneumonia as a complication. * CT Sinus: For chronic or recurrent sinusitis associated with allergic rhinitis, to evaluate for structural abnormalities or inflammatory changes. - Advanced: * High-Resolution CT (HRCT) Chest: In suspected allergic bronchopulmonary aspergillosis (ABPA). * Barium Swallow/Endoscopy with Biopsy: For eosinophilic gastrointestinal disorders (e.g., eosinophilic esophagitis), to assess inflammation and strictures.

Management

Acute Management

Prioritize ABCs (Airway, Breathing, Circulation) in acute allergic reactions. - Immediate (Anaphylaxis): 1. Epinephrine (Adrenaline): First-line and most critical treatment. Administer intramuscularly (IM) into the mid-anterolateral thigh. * Adults: 0.3-0.5 mg IM (e.g., EpiPen, Auvi-Q, Adrenaclick 0.3 mg or 0.5 mg auto-injector). * Children: 0.01 mg/kg IM, max 0.3 mg (e.g., EpiPen Jr. 0.15 mg auto-injector for 10-25 kg, 0.3 mg for >25 kg). * Repeat every 5-15 minutes if symptoms persist or recur. 2. Call for help/Emergency Services: Activate emergency medical system immediately. 3. Positioning: Lay patient flat with legs elevated (Trendelenburg) if hypotensive; if vomiting or airway compromise, semi-recumbent or lateral recumbent. Pregnant patients should be on left lateral decubitus. 4. Oxygen: Administer 100% oxygen via non-rebreather mask (10-15 L/min). 5. Fluid Resuscitation: Rapid IV bolus of isotonic crystalloids (e.g., 0.9% NaCl) for hypotension. * Adults: 500-1000 mL rapidly. * Children: 20 mL/kg rapidly. * Repeat as needed. - Stabilization: * Antihistamines: H1-blockers (e.g., diphenhydramine IV/IM/PO, cetirizine PO) for cutaneous symptoms (urticaria, pruritus). Not first-line for anaphylaxis, do not treat life-threatening symptoms. * Corticosteroids: Systemic (e.g., methylprednisolone IV, prednisone PO) to prevent protracted or biphasic reactions. Administer after epinephrine. * Bronchodilators: Inhaled beta-agonists (e.g., albuterol/salbutamol) for bronchospasm/wheezing not responsive to epinephrine alone. * Vasopressors: If hypotension persists despite epinephrine and fluids (e.g., norepinephrine, vasopressin infusions in ICU setting). - Emergency: * Airway Management: Endotracheal intubation for impending or complete airway obstruction (laryngeal edema). * CPR: If cardiac arrest occurs.

Chronic Management

Focus on allergen avoidance, pharmacotherapy, and immunotherapy. - Pharmacological: * Antihistamines (H1-blockers): * First-generation (sedating): Diphenhydramine, hydroxyzine. Used for acute symptoms, but limited by sedation and anticholinergic effects. * Second-generation (non-sedating): Loratadine, cetirizine, fexofenadine, desloratadine, levocetirizine. Preferred for chronic management of allergic rhinitis, urticaria. Daily dosing. * Corticosteroids: * Intranasal corticosteroids (INCS): Fluticasone, mometasone, budesonide. First-line for allergic rhinitis. Daily use. * Inhaled corticosteroids (ICS): Fluticasone, budesonide, beclomethasone. First-line for persistent asthma. * Topical corticosteroids: For atopic dermatitis (e.g., triamcinolone, betamethasone, clobetasol). * Systemic corticosteroids: Reserved for severe exacerbations, short courses. * Leukotriene Receptor Antagonists (LTRAs): Montelukast, zafirlukast. Used for allergic rhinitis and mild persistent asthma. Less effective than INCS for rhinitis or ICS for asthma. * Mast Cell Stabilizers: Cromolyn sodium (nasal spray, eye drops, nebulized). Prevents mediator release from mast cells. Less potent than corticosteroids, but useful as an alternative or add-on. * Biologics: Monoclonal antibodies targeting specific components of the allergic inflammatory cascade. * Omalizumab (anti-IgE): For moderate-to-severe persistent allergic asthma inadequately controlled by ICS, and chronic spontaneous urticaria. * Mepolizumab, Reslizumab, Benralizumab (anti-IL-5): For severe eosinophilic asthma. * Dupilumab (anti-IL-4Rα): For moderate-to-severe atopic dermatitis, severe eosinophilic asthma, chronic rhinosinusitis with nasal polyps, eosinophilic esophagitis. * Immunosuppressants: For severe, refractory allergic conditions (e.g., cyclosporine, methotrexate for severe atopic dermatitis or chronic urticaria). - Non-pharmacological: * Allergen Avoidance: Key for environmental, food, and drug allergies. * Dust mites: Encase mattresses/pillows, wash bedding hot (>55°C), maintain low humidity (<50%), remove carpets. * Pets: Remove pet, if not possible, keep out of bedroom, HEPA filters, frequent washing. * Pollen: Keep windows closed, use A/C, avoid outdoor activities during high pollen counts. * Food Allergy: Strict avoidance of confirmed food allergens. Education on reading labels, cross-contamination. * Patient Education: Anaphylaxis action plan, proper use of auto-injectors, medication adherence. * Environmental Controls: Air filters, dehumidifiers, mold remediation. * Skin Care (Atopic Dermatitis): Regular moisturizing, lukewarm baths, gentle cleansers, wet wraps. * Nasal Saline Rinses: For allergic rhinitis to clear allergens and mucus. - Monitoring: * Allergic Rhinitis/Asthma: Symptom diaries, Peak Flow monitoring (asthma), follow-up visits to assess control and adjust therapy (e.g., GINA guidelines for asthma). * Food Allergy: Serial sIgE testing to monitor potential for resolution; re-evaluation with OFC. * Immunotherapy: Monitor for local and systemic reactions, symptom improvement.

Treatment Algorithms

General principles; specific guidelines vary by condition (e.g., ARIA for rhinitis, GINA for asthma). - Allergic Rhinitis: * First-Line: Allergen avoidance + Intranasal Corticosteroids (INCS). * Second-Line: Add oral antihistamines (non-sedating), LTRAs, or nasal antihistamines. If severe, consider short course oral corticosteroids (rarely). * Refractory Cases: Allergen Immunotherapy (AIT). Consider combination therapies, re-evaluation of diagnosis. - Allergic Asthma: * First-Line (GINA): Reliever (low-dose ICS-formoterol as needed OR SABA as needed with regular low-dose ICS) for mild asthma. Daily low-dose ICS-formoterol for moderate. * Second-Line: Step-up ICS dose, add LTRA, or add long-acting beta-agonist (LABA). * Refractory Cases: High-dose ICS/LABA, add LTRA, refer for biologics (Omalizumab, anti-IL-5s, Dupilumab), consider bronchial thermoplasty. - Chronic Spontaneous Urticaria (CSU): * First-Line: Second-generation non-sedating H1-antihistamine (regular daily dose). * Second-Line: Up-dose H1-antihistamine up to 4 times standard dose. * Third-Line: Add Omalizumab. * Refractory Cases: Add Cyclosporine.

Complications

Common:
- Allergic Rhinitis: Sinusitis, otitis media, nasal polyps, sleep disturbance, impaired quality of life, impaired concentration.
- Asthma: Exacerbations, respiratory infections, poor sleep, reduced physical activity.
- Atopic Dermatitis: Skin infections (bacterial, viral, fungal), eczema herpeticum, sleep disturbance, psychosocial impact.
- Food Allergy: Nutritional deficiencies (especially with multiple food allergies or restrictive diets), growth impairment, anxiety, social isolation.
Serious:
- Anaphylaxis: Potentially fatal, leading to circulatory shock, respiratory arrest.
- Severe Asthma Exacerbations: Respiratory failure, hospitalization, intubation, death.
- Angioedema of the Upper Airway: Airway obstruction, asphyxiation.
- Drug Reactions: DRESS syndrome, Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN) - these are severe cutaneous adverse reactions, some are immune-mediated and life-threatening.
Long-term:
- Chronic Rhinosinusitis: Persistent inflammation, nasal obstruction.
- Airway Remodeling: In uncontrolled asthma, leading to irreversible airflow limitation.
- Psychosocial Impact: Anxiety, depression, impaired social functioning, reduced quality of life due to chronic symptoms, fear of reactions, or restrictive diets.
- Eosinophilic Esophagitis (EoE): Esophageal strictures, dysphagia, food impaction.

Prognosis

Factors Affecting Outcome:
- Severity of Initial Reaction: More severe initial reactions often predict future severe reactions (e.g., anaphylaxis).
- Nature of Allergen: Allergy to certain allergens (e.g., peanut, tree nuts, seafood) is less likely to be outgrown than to others (e.g., milk, egg).
- Age of Onset: Allergies developing later in life tend to be more persistent.
- Comorbidities: Presence of multiple atopic conditions (allergic march), or other chronic diseases, can worsen prognosis.
- Adherence to Treatment/Avoidance: Good adherence improves outcomes.
- Access to Care/Epinephrine: Timely access to emergency treatment significantly improves survival in anaphylaxis.
Survival: Excellent for most allergic conditions with proper management. Anaphylaxis fatalities are rare but possible, often due to delayed epinephrine administration or underlying cardiovascular/respiratory disease.
Quality of Life: Allergic diseases can significantly impair quality of life due to chronic symptoms, sleep disturbance, school/work absenteeism, activity limitation, and psychological burden. Effective management is crucial for improving QoL.

Prevention

Primary (Preventing Initial Occurrence):
- Dietary Introduction: For food allergy, early introduction of common allergenic foods (peanut, egg, milk, etc.) from 4-6 months of age, particularly in high-risk infants (those with severe eczema or egg allergy), can reduce the risk.
- Gut Microbiome Modulation: Research ongoing on prebiotics/probiotics, but no definitive guidelines yet for allergy prevention.
- Avoidance of Tobacco Smoke: During pregnancy and early childhood.
- Breastfeeding: Exclusive breastfeeding for the first 4-6 months is encouraged for general health, but its specific role in preventing allergy is debated.
Secondary (Early Detection and Intervention):
- Early Diagnosis: Prompt identification of allergies allows for appropriate avoidance and management strategies.
- Environmental Control Measures: Reducing exposure to known environmental allergens (dust mites, pet dander, pollen) once sensitized.
- Allergen Immunotherapy (AIT): Can prevent the development of new sensitizations and reduce progression of allergic rhinitis to asthma in some cases.
Tertiary (Preventing Complications/Exacerbations):
- Anaphylaxis Action Plan: For individuals with severe allergies, detailing recognition of symptoms and immediate treatment (epinephrine auto-injector).
- Epinephrine Auto-injector Prescription: For all patients with a history of anaphylaxis or significant risk.
- Allergen Avoidance: Strict avoidance of identified triggers.
- Regular Pharmacotherapy: Consistent use of controller medications (e.g., ICS for asthma, INCS for rhinitis) to prevent exacerbations.
- Allergen Immunotherapy (AIT): Reduces severity of symptoms and medication use in allergic rhinitis and asthma.

Special Populations

Pediatric:
- Dosing: All medication doses are weight-based (e.g., epinephrine 0.01 mg/kg).
- Food Allergy: More common than in adults. Greater likelihood of outgrowing milk, egg, soy, wheat allergies. Emphasis on early allergen introduction. Impact on growth/nutrition is critical.
- Asthma: Viral-induced wheezing is common. Challenges with adherence to medication.
- Atopic Dermatitis: Often starts in infancy, typically affects face, scalp, extensors in infants; flexural areas in older children.
Geriatric:
- Polypharmacy: Increased risk of drug interactions and side effects. Consider reduced metabolism and clearance.
- Comorbidities: Cardiovascular disease, diabetes, glaucoma, benign prostatic hyperplasia can influence medication choices (e.g., anticholinergic effects of first-gen antihistamines, beta-blockers interactions with epinephrine).
- Cognitive Impairment: Challenges with medication adherence or using auto-injectors.
- Reduced Immune Response: AIT may be less effective.
Pregnancy:
- Medication Safety: Prioritize medications with established safety profiles.
  - Allergic Rhinitis: Saline rinses, INCS (budesonide preferred), second-generation antihistamines (loratadine, cetirizine preferred).
  - Asthma: Uncontrolled asthma poses greater risk to fetus than medications. Continue ICS. Oral corticosteroids for severe exacerbations are generally safe.
  - Anaphylaxis: Epinephrine is safe and essential. No contraindication due to pregnancy. Maintain maternal blood pressure and oxygenation.
Comorbidities:
- Asthma and Allergic Rhinitis: Often co-exist; treating rhinitis improves asthma control. "One airway, one disease."
- Atopic Dermatitis and Food Allergy: Strong association. Severe eczema is a risk factor for food allergy.
- Cardiovascular Disease: Caution with epinephrine in patients with severe CAD; however, risks of untreated anaphylaxis outweigh risks of epinephrine.
- Mental Health: Anxiety and depression are common in chronic allergic conditions, requiring holistic management.

Clinical Pearls

Diagnostic:
- History is King: A thorough history is often more valuable than lab tests for allergy diagnosis. Tests confirm sensitization, not always clinical allergy.
- Skin Prick Test (SPT) vs. Specific IgE (sIgE): SPT is generally more sensitive and immediate. sIgE is useful when SPT is contraindicated. A negative test makes IgE-mediated allergy unlikely, but a positive test alone does not confirm clinical allergy.
- Oral Food Challenge (OFC): The gold standard for confirming food allergy and assessing resolution, but requires careful planning and supervision due to risk.
- Tryptase for Anaphylaxis: A normal tryptase does not rule out anaphylaxis, especially if collected too early or too late. Elevated levels support diagnosis but are not required.
Treatment:
- Epinephrine First: Always administer epinephrine immediately for suspected anaphylaxis. Delay is associated with increased mortality. Antihistamines and steroids are secondary.
- Avoidance vs. Immunotherapy: Strict avoidance is essential, but for unavoidable environmental allergens, Allergen Immunotherapy (AIT) is the only disease-modifying treatment.
- Chronic Urticaria: Rule out underlying causes. Up-dosing H1-antihistamines is common practice before adding biologics.
- Food Allergy Management: Focus on avoidance and emergency preparedness. Nutritional counseling and regular re-evaluation are crucial.
Pitfalls:
- Over-reliance on Blood Tests: Positive sIgE tests without correlating clinical history can lead to unnecessary avoidance and anxiety.
- Misdiagnosis of Drug Allergy: Many non-allergic adverse drug reactions are labeled as "allergies," leading to avoidance of effective medications. Formal drug allergy evaluation is often warranted.
- Under-treatment of Anaphylaxis: Not administering epinephrine due to mild initial symptoms or fear of side effects.
- Ignoring Co-factors: For food allergy, co-factors like exercise, NSAIDs, or alcohol can lower the reaction threshold and must be considered.
- Lack of Anaphylaxis Action Plan: Patients with severe allergies must have a clear action plan and readily available epinephrine auto-injectors.

Recent Updates

Guidelines:
- NIAID/FAAN 2020 Guidelines for Anaphylaxis: Emphasize early epinephrine, prompt recognition, and patient education.
- GINA (Global Initiative for Asthma) 2023 Update: Continued emphasis on ICS-containing regimens for all asthma severities, including mild. SABA monotherapy is no longer recommended.
- ARIA (Allergic Rhinitis and its Impact on Asthma) 2023 Update: Reinforce INCS as first-line for moderate-severe allergic rhinitis.
- LEAP and EAT Studies (for Food Allergy Prevention): Pivotal studies demonstrating the benefit of early introduction of allergenic foods to reduce the risk of food allergy development. These findings have led to widespread changes in infant feeding guidelines globally.
Evidence:
- Emerging Biologics: Expanding indications for biologics (e.g., Dupilumab for multiple atopic conditions, new biologics for peanut allergy - Palforzia).
- Oral Immunotherapy (OIT): Increasingly used for food allergy; while not a cure, it aims to raise the reaction threshold to accidental exposures. Requires careful supervision.
- Sublingual Immunotherapy (SLIT): Tablet formulations approved for grass, ragweed, and dust mite allergies provide a convenient alternative to subcutaneous injections.
Controversies:
- Optimal Duration of AIT: While generally 3-5 years, optimal duration for different allergens and patient groups is debated.
- Role of Microbiome Modification: While promising, definitive interventions for allergy prevention based on microbiome remain elusive.
- Prevention of Food Allergy: Debates on the exact timing, method, and range of allergens for early introduction.
- Allergy Testing in Absence of Symptoms: Routine screening for allergies is generally not recommended unless clinically indicated by symptoms.

Key References

Guidelines:
- NIAID/FAAN: National Institute of Allergy and Infectious Diseases / Food Allergy & Anaphylaxis Network (Guidelines for the Diagnosis and Management of Food Allergy in the United States).
- GINA: Global Initiative for Asthma (Global Strategy for Asthma Management and Prevention).
- ARIA: Allergic Rhinitis and its Impact on Asthma (Pocket Guide for Allergen Immunotherapy, ARIA-EAACI recommendations).
- WAO: World Allergy Organization (Anaphylaxis Guidelines, Urticaria Guidelines).
- EAACI: European Academy of Allergy and Clinical Immunology (Numerous position papers and guidelines on various allergic diseases).
- AAAAI/ACAAI: American Academy of Allergy, Asthma & Immunology / American College of Allergy, Asthma & Immunology (Practice Parameters).
Studies:
- LEAP Study (Learning Early About Peanut Allergy): Du Toit G, et al. Randomized trial of peanut consumption in infants at risk for peanut allergy. N Engl J Med. 2015.
- EAT Study (Enquiring About Tolerance): Perkin MR, et al. Randomized trial of early introduction of six allergenic foods and risk of food allergy. N Engl J Med. 2016.
- TENOR Study (The Epidemiology of Niacin in the Treatment of Osteoarthritis): Large observational study providing insights into severe asthma.
Reviews:
- Leading medical textbooks on allergy and immunology (e.g., Middleton's Allergy, Adkinson's Allergy, Principles and Practice).
- Review articles in reputable journals such as The Journal of Allergy and Clinical Immunology (JACI), Allergy, Lancet, New England Journal of Medicine.