Hypersensitivity Reactions
An Interactive Study Guide & Quiz
Classification Table
| Type | Mechanism | Key Diseases | Pathogenesis | Examples of Target Antigens/Allergens |
|---|---|---|---|---|
| Type I (Immediate) |
IgE-mediated mast cell degranulation | Allergic rhinitis Asthma Anaphylaxis Food allergies Atopic dermatitis |
IgE binds allergen → mast cell activation → release of histamine, leukotrienes | Pollen, peanuts, bee venom, penicillin |
| Type II (Cytotoxic) |
IgG/IgM vs. cell-surface antigens | Autoimmune hemolytic anemia Graves' disease Myasthenia gravis Pemphigus vulgaris |
Antibody opsonization → phagocytosis Antibody-mediated cell dysfunction Complement activation |
RBC membrane proteins TSH receptor AChR Desmoglein |
| Type III (Immune Complex) |
Deposition of antigen-antibody complexes | Systemic lupus erythematosus (SLE) Rheumatoid arthritis Serum sickness Post-strep glomerulonephritis |
Complexes deposit in tissues → complement activation → neutrophil recruitment → inflammation | Nuclear antigens (SLE) IgG (RA) Foreign proteins (serum sickness) Streptococcal antigens |
| Type IV (Delayed) |
T cell-mediated inflammation | Multiple sclerosis Contact dermatitis Type 1 diabetes Psoriasis Tuberculin reaction |
CD4+ Th1/Th17 cells → cytokine release → macrophage activation CD8+ CTL → direct cytotoxicity |
Myelin basic protein (MS) Poison ivy urushiol Insulin/glutamic acid decarboxylase (T1D) Unknown self-antigens (psoriasis) |
Key Distinctions
1. Type I
- Onset: Seconds–minutes.
- Hallmark: IgE, eosinophilia, anaphylaxis.
- Treatment: Antihistamines, epinephrine.
2. Type II
- Onset: Hours–days.
- Hallmark: Antibodies targeting cell surfaces or ECM.
- Diagnosis: Coombs test.
3. Type III
- Onset: 1–3 weeks.
- Hallmark: Vasculitis, glomerulonephritis.
- Diagnosis: Anti-dsDNA, rheumatoid factor.
4. Type IV
- Onset: 48–72 hours.
- Hallmark: Granuloma formation, epidermal blistering.
- Diagnosis: Patch testing, T cell assays.
Notable Overlaps
- Rheumatoid arthritis: Primarily Type III, with Type IV components.
- Goodpasture's syndrome: Type II (anti-basement membrane antibodies).
- Hashimoto's thyroiditis: Type II (antibody-mediated) + Type IV (T cell infiltration).
⚠️ Clinical Insight: Some diseases (e.g., celiac disease) involve mixed hypersensitivity (Type IV + antibody-mediated damage).
Test Your Knowledge
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