Allergies and Hypersensitivity
"Allergies" and "hypersensitivity reactions" are terms that describe an overreaction of the immune system to substances that are typically harmless to most people. While often used interchangeably, "hypersensitivity reaction" is a broader medical term that includes allergies, but also other immune-mediated reactions that may not involve the typical allergic mechanisms.
Allergies
Allergy is a specific type of hypersensitivity reaction (Type I Hypersensitivity) that involves an immediate, exaggerated immune response to a normally harmless substance (an allergen). This reaction is mediated primarily by IgE antibodies.
Key Characteristics of Allergies:
Cause
Exposure to specific allergens (e.g., pollen, dust mites, pet dander, certain foods like peanuts or shellfish, insect stings, medications like penicillin).
Mechanism (Type I Hypersensitivity):
- Sensitization: First exposure to an allergen causes the immune system to produce specific IgE antibodies.
- These IgE antibodies attach to mast cells (in tissues) and basophils (in blood).
- Re-exposure: Upon subsequent exposure to the same allergen, the allergen binds to the IgE on mast cells/basophils.
- This triggers the rapid release of inflammatory mediators (like histamine, leukotrienes, prostaglandins) from these cells.
Immune Cell Production
Produces lymphocytes, which are critical components of the immune system.
Symptoms
- Skin: Hives (urticaria), eczema, itching.
- Respiratory: Sneezing, runny nose, nasal congestion, itchy eyes (allergic rhinitis/hay fever), wheezing, shortness of breath, asthma.
- Gastrointestinal: Nausea, vomiting, diarrhea, abdominal pain (food allergies).
- Systemic (Anaphylaxis): A severe, life-threatening allergic reaction involving multiple body systems, leading to difficulty breathing, a sudden drop in blood pressure, swelling of the face/throat, dizziness, and loss of consciousness.
Diagnosis
- Skin Prick Test: Small amounts of allergens are pricked into the skin to observe a localized allergic reaction.
- Specific IgE Blood Tests (RAST, ImmunoCAP): Measure the level of IgE antibodies in the blood directed against specific allergens.
- Oral Food Challenge: Supervised ingestion of a suspected food allergen (gold standard for food allergies).
Treatment
Avoidance of allergens, antihistamines, corticosteroids, epinephrine (for anaphylaxis), allergen immunotherapy (allergy shots).
Hypersensitivity Reactions (Broader Term)
Hypersensitivity reaction is a broader immunological term referring to undesirable reactions produced by the normal immune system, including allergies. These reactions are often damaging, uncomfortable, or sometimes fatal. They are classified into four main types (Gell and Coombs classification) based on the immune mechanism involved:
Type I: Immediate Hypersensitivity (Allergy)
Mechanism
IgE-mediated release of histamine and other mediators from mast cells and basophils.
Examples
Aergic rhinitis, asthma, anaphylaxis, food allergies.
Onset
Minutes to hours.
Type II: Cytotoxic Hypersensitivity
Mechanism
IgG or IgM antibodies bind to antigens on the surface of cells, leading to cell destruction (cytotoxicity) primarily through complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC).
Examples
- Transfusion Reactions: Reaction to incompatible blood transfusions (e.g., ABO incompatibility).
- Hemolytic Disease of the Fetus and Newborn (HDFN): Rh incompatibility between mother and fetus.
- Autoimmune Hemolytic Anemia (AIHA): Antibodies attack red blood cells.
- Drug-induced hemolytic anemia.
Onset
Hours to days.
Type III: Immune Complex-Mediated Hypersensitivity
Mechanism
Formation of antigen-antibody (IgG or IgM) immune complexes that deposit in tissues (e.g., blood vessel walls, kidney glomeruli, joints), triggering inflammation and tissue damage.
Examples
- Serum Sickness: Reaction to certain medications or antitoxins.
- Systemic Lupus Erythematosus (SLE): Immune complexes contribute to kidney damage, arthritis, etc.
- Rheumatoid Arthritis (RA): Immune complexes in joints.
- Post-streptococcal Glomerulonephritis.
Onset
Days to weeks.
Type IV: Delayed-Type Hypersensitivity (Cell-Mediated)
Mechanism
Mediated by T-cells (specifically T-helper cells and cytotoxic T-lymphocytes), not antibodies. The reaction takes time to develop as T-cells need to migrate to the site of antigen exposure.
Examples
- Contact Dermatitis: Skin rash from contact with poison ivy, nickel, latex.
- Tuberculin Skin Test (PPD test): For tuberculosis exposure.
- Graft-versus-Host Disease (GVHD): In organ/bone marrow transplantation.
- Some autoimmune diseases (e.g., Type 1 Diabetes, Multiple Sclerosis).
Onset
24-72 hours (delayed).
Clinical Importance:
Understanding the distinction between allergies and other hypersensitivity reactions, and identifying the specific type, is critical for:
Accurate Diagnosis
Guiding the correct diagnostic tests.
Effective Treatment
Selecting appropriate medications (e.g., antihistamines for Type I, corticosteroids for inflammation, specific immunosuppressants for autoimmune conditions).
Prevention
Advising on avoidance strategies or desensitization.
Patient Safety
Preventing severe or life-threatening reactions, especially in transfusion medicine or drug administration.