Hypersensitivity reactions are exaggerated or inappropriate immune responses that result in tissue damage or disease. These responses are typically classified into four types, remembered easily using the mnemonic ACID:
- A - Anaphylactic (Type I)
- C - Cytotoxic (Type II)
- I - Immune Complex-mediated (Type III)
- D - Delayed-type (Type IV)
Whether you’re a medical student prepping for exams or a health enthusiast curious about immune responses, this guide simplifies the complexities of immunopathology with real-life examples, mechanisms, and memory aids.
What Is Hypersensitivity?
Hypersensitivity is an abnormal immune reaction to an otherwise harmless antigen, also known as an allergen. It’s an overreaction of the immune system that can result in a range of outcomes—from mild rashes to life-threatening conditions like anaphylactic shock or autoimmune diseases.
These responses require prior sensitization, meaning the body must have encountered the antigen before.
Type I Hypersensitivity: Anaphylactic Reaction (A in ACID)
Mechanism
Type I hypersensitivity involves IgE antibodies produced in response to an allergen. Upon re-exposure, these IgE antibodies bind to mast cells and basophils, triggering the release of histamine, prostaglandins, and leukotrienes.
Examples
- Asthma
- Allergic rhinitis (hay fever)
- Anaphylaxis due to peanut or penicillin allergy
- Urticaria (hives)
- Eczema
Symptoms
- Itching
- Swelling (angioedema)
- Bronchoconstriction
- Hypotension (in anaphylaxis)
- Wheezing
Diagnostic Tests
- Skin prick test
- Serum IgE levels
Treatment
- Epinephrine (first-line in anaphylaxis)
- Antihistamines
- Corticosteroids
- Avoidance of known allergens
Type II Hypersensitivity: Cytotoxic Reaction (C in ACID)
Mechanism
This involves IgG or IgM antibodies binding directly to antigens on host cell surfaces, leading to:
- Complement activation
- Cell lysis
- Opsonization and phagocytosis
Examples
- Hemolytic anemia
- Goodpasture’s syndrome
- Rheumatic fever
- Myasthenia gravis
- Grave’s disease
- Transfusion reactions
- Hemolytic disease of the newborn (Erythroblastosis fetalis)
Symptoms
- Fatigue (due to hemolysis)
- Hematuria (in Goodpasture's)
- Muscle weakness (in Myasthenia gravis)
- Hyperthyroidism symptoms (in Grave’s disease)
Diagnostic Tests
- Coombs test
- Anti-AChR antibodies
- ELISA for specific autoantibodies
Treatment
- Immunosuppressants
- Plasmapheresis
- Corticosteroids
- Avoidance of triggering antigens
Type III Hypersensitivity: Immune Complex-Mediated (I in ACID)
Mechanism
Antigen-antibody complexes (mainly IgG) deposit in tissues, activating the complement system and leading to inflammation and tissue damage.
Examples
- Systemic lupus erythematosus (SLE)
- Post-streptococcal glomerulonephritis
- Serum sickness
- Hypersensitivity pneumonitis (farmer’s lung)
Symptoms
- Fever
- Arthralgia
- Rash
- Proteinuria and hematuria (in nephritis)
- Respiratory symptoms (in pneumonitis)
Diagnostic Tests
- ANA (Anti-nuclear antibody)
- Anti-dsDNA antibodies
- Low complement levels (C3, C4)
- Renal biopsy (for immune complex deposition)
Treatment
- NSAIDs
- Corticosteroids
- Immunosuppressants
- Plasmapheresis
Type IV Hypersensitivity: Delayed-Type Reaction (D in ACID)
Mechanism
This type is T-cell mediated, specifically CD4+ T-helper cells and CD8+ cytotoxic T cells. It takes 48–72 hours to manifest after exposure.
Examples
- Tuberculin skin test (Mantoux test)
- Contact dermatitis (e.g., poison ivy, nickel allergy)
- Multiple sclerosis
- Type 1 diabetes mellitus
- Hashimoto thyroiditis
Symptoms
- Erythema
- Induration
- Rash
- Destruction of tissue (in autoimmune diseases)
Diagnostic Tests
- Patch test (for dermatitis)
- Mantoux test (for TB)
- T-cell assays
Treatment
- Corticosteroids
- Immunosuppressants
- Topical treatments (for skin conditions)
ACID Mnemonic: Quick Recap Table
| Type | Name | Mechanism | Antibodies/Cells | Key Examples |
|---|---|---|---|---|
| I | Anaphylactic | IgE-mediated mast cell degranulation | IgE | Asthma, food allergy, anaphylaxis |
| II | Cytotoxic | IgG/IgM against host cells | IgG, IgM | Grave’s disease, Goodpasture's, transfusions |
| III | Immune Complex-Mediated | Immune complexes + complement | IgG | SLE, serum sickness, glomerulonephritis |
| IV | Delayed-Type (Cell-Mediated) | T-cell mediated, delayed onset | T-Cells | TB test, contact dermatitis, Type 1 diabetes |
Clinical Case Studies
Case 1: Peanut Allergy (Type I)
A 10-year-old boy experiences swelling and wheezing within minutes of eating a peanut butter sandwich. He’s administered epinephrine and stabilized. Diagnosis: Anaphylactic shock.
Case 2: Blood Transfusion Reaction (Type II)
A patient receives the wrong blood type during surgery and develops fever, chills, and hematuria. Direct Coombs test is positive. Diagnosis: Acute hemolytic transfusion reaction.
Case 3: Post-Streptococcal Glomerulonephritis (Type III)
A 7-year-old presents with hematuria and hypertension 2 weeks after a sore throat. Lab tests show low complement levels. Diagnosis: Immune complex glomerulonephritis.
Case 4: Contact Dermatitis (Type IV)
A woman develops an itchy, red rash 48 hours after wearing a new nickel bracelet. Diagnosis: Allergic contact dermatitis.
Mnemonic-Based Learning Tip
Use this ACID mnemonic while preparing for exams:
- A – Asthma, Anaphylaxis
- C – Cytotoxic, Coombs test
- I – Immune complexes, Inflammation
- D – Delayed, Dermatitis
It’s especially helpful in recalling mechanisms, immune mediators, and clinical correlations.
FAQs on Hypersensitivity Reactions
Q1. Which hypersensitivity type is IgE-mediated?
Type I hypersensitivity is IgE-mediated.
Q2. Which reactions are antibody-mediated?
Type II (Cytotoxic) and Type III (Immune Complex) reactions involve antibodies.
Q3. What type is autoimmune hemolytic anemia?
It’s a Type II hypersensitivity reaction.
Q4. Why is Type IV called delayed?
Because the response takes 48–72 hours to develop after exposure due to T-cell mediation.
Q5. Is asthma a Type I or Type IV reaction?
Asthma is a Type I hypersensitivity reaction.
Conclusion
Understanding hypersensitivity reactions is critical for diagnosing and treating a range of allergic and autoimmune diseases. The ACID mnemonic is an invaluable tool for quickly recalling the mechanisms, mediators, and examples of each hypersensitivity type.
🩺 More Helpful Medical Guides:
- Exercise Guide for Diabetics (FIT Mnemonic)
- Alcoholic Liver Disease (DAMP PAGES Mnemonic)
- Heat Stroke Symptoms and Management (TIRED)
- Abdominal Pain Differential Diagnosis (ABDOMINAL)
- Macrocytic Anaemia Causes (FAT RBC Mnemonic)
- Types of Hypersensitivity Reactions (ACID Mnemonic)
- Hyperkalemia ECG Changes and Management
- Drugs That Cause Porphyria (ABCDE Mnemonic)
- Causes and Evaluation of Haemoptysis
- Enuresis (Bedwetting): Types and Treatment
- Asthma Clinical Features and Management
