Hypersensitivity refers to an exaggerated, inappropriate, or damaging immune response that occurs when the immune system reacts to an antigen or other stimulus. Although immune responses normally protect the body against infections and harmful substances, certain reactions can injure healthy tissues and contribute to disease.
Hypersensitivity reactions may occur in response to environmental allergens, medications, microbial antigens, or the body’s own components. Their effects range from mild symptoms, such as itching and sneezing, to severe systemic reactions that can become life-threatening.
The traditional classification proposed by immunologist Philip Gell and Robin Coombs divides hypersensitivity into four major types: Type I, Type II, Type III, and Type IV. These categories are distinguished by their underlying immune mechanisms, the components involved, and the time course of the reaction.
- Type I hypersensitivity: Immediate reactions involving IgE antibodies, mast cells, and mediators such as histamine.
- Type II hypersensitivity: Antibody-mediated reactions involving IgG or IgM directed against cell-surface or tissue antigens.
- Type III hypersensitivity: Reactions caused by immune-complex formation and deposition in tissues.
- Type IV hypersensitivity: Delayed reactions primarily mediated by sensitized T lymphocytes rather than antibodies.
These four types is important in immunology because they explain the mechanisms behind many allergic disorders, immune-mediated tissue injuries, and inflammatory diseases. Studying their characteristic mechanisms and examples also helps students distinguish between conditions that may produce similar symptoms but develop through different immune pathways.
Classification of Hypersensitivity Reactions
Hypersensitivity reactions are traditionally classified into four major types according to the immune mechanisms responsible for tissue injury. This classification, known as the Gell and Coombs classification, is widely used in immunology to understand allergic reactions, antibody-mediated damage, immune-complex diseases, and T-cell-mediated inflammation.
The four types differ in their immune components, mechanisms of tissue injury, and typical clinical examples.
1. Type I Hypersensitivity — Immediate Type
Type I hypersensitivity is an immediate allergic reaction mediated by immunoglobulin E (IgE). During the sensitization phase, exposure to an allergen stimulates the production of allergen-specific IgE, which binds to receptors on mast cells and basophils.
On subsequent exposure, the allergen cross-links the bound IgE, triggering the release of histamine and other inflammatory mediators.
Examples:
- Allergic rhinitis
- Allergic asthma in susceptible individuals
- Urticaria (hives)
- Anaphylaxis
2. Type II Hypersensitivity — Antibody-Mediated Type
Type II hypersensitivity occurs when IgG or IgM antibodies recognize antigens on cell surfaces or within tissues. Antibody binding may activate complement, promote phagocytosis, recruit immune cells, or interfere with receptor function.
The resulting effects depend on the target antigen and the mechanism involved.
Examples:
- Autoimmune haemolytic anaemia
- Haemolytic disease of the fetus and newborn
- Goodpasture disease
- Graves disease, in which antibodies stimulate the thyroid-stimulating hormone receptor
3. Type III Hypersensitivity — Immune Complex-Mediated Type
Type III hypersensitivity develops when antigen–antibody complexes form and deposit in tissues or blood vessels. These deposits can activate complement and recruit inflammatory cells, leading to tissue injury.
Examples:
- Systemic lupus erythematosus (SLE)
- Serum sickness
- Arthus reaction
- Some forms of immune-complex glomerulonephritis
4. Type IV Hypersensitivity — Delayed-Type
Type IV hypersensitivity is mediated primarily by sensitized T lymphocytes rather than antibodies. After recognizing an antigen, these cells release cytokines, recruit inflammatory cells, or directly damage target cells.
The reaction generally develops over 24–72 hours, although the timing varies with the specific condition and exposure.
Examples:
- Contact dermatitis caused by substances such as nickel
- Tuberculin skin test reaction
- Granulomatous inflammation associated with tuberculosis
- Type 1 diabetes mellitus, involving T-cell-mediated destruction of pancreatic beta cells
5. Comparison of the Four Types
| Feature | Type I | Type II | Type III | Type IV |
|---|---|---|---|---|
| Common name | Immediate | Antibody-mediated | Immune complex-mediated | Delayed-type |
| Main immune component | IgE | IgG or IgM | Antigen–antibody complexes | T lymphocytes |
| Major mechanism | Mast-cell activation and mediator release | Antibody-dependent cellular injury or receptor effects | Immune-complex deposition and inflammation | T-cell cytokines or direct cytotoxicity |
| Typical onset | Minutes; late-phase reactions may follow | Variable | Variable, depending on the disease | Commonly 24–72 hours |
| Representative example | Anaphylaxis | Autoimmune haemolytic anaemia | Serum sickness | Contact dermatitis |
6. Importance of the Classification
This classification helps explain why different immune reactions cause different patterns of disease. For example, Type I reactions involve IgE and mast cells, whereas Type IV reactions rely primarily on T cells. Type II and Type III reactions both involve antibodies, but Type II targets cell-associated or tissue antigens, while Type III involves deposited immune complexes.
Recognizing the underlying mechanism helps clinicians interpret symptoms, select appropriate investigations, and determine suitable treatment strategies.
Type I Hypersensitivity — Immediate Hypersensitivity
Type I hypersensitivity is an immediate allergic reaction mediated by immunoglobulin E (IgE), mast cells, and basophils. It occurs when the immune system responds excessively to an allergen, such as pollen, dust mite proteins, certain foods, or insect venom. The release of inflammatory mediators produces symptoms that range from localized itching and sneezing to severe systemic reactions.
Common examples include allergic rhinitis, some forms of allergic asthma, urticaria, food allergy, and anaphylaxis.
1. Mechanism of Type I Hypersensitivity
Type I reactions generally involve two stages: sensitization and subsequent allergen exposure.
A. Sensitization phase
During the first clinically relevant exposure to an allergen:
- Antigen-presenting cells take up and process the allergen.
- They present allergen-derived peptides to suitable CD4+ T cells.
- In susceptible individuals, a type 2 immune response develops, involving Th2 cells and related signals.
- Interleukin-4 (IL-4) and interleukin-13 (IL-13) promote B-cell class switching to IgE.
- Allergen-specific IgE binds to high-affinity FcεRI receptors on mast cells and basophils.
This process sensitizes the individual. The first exposure does not necessarily cause obvious allergic symptoms.
B. Re-exposure phase
When the sensitized individual encounters the same allergen again:
- The allergen binds and cross-links allergen-specific IgE on mast cells or basophils.
- These cells become activated and release inflammatory mediators.
- Histamine, leukotrienes, prostaglandins, and other mediators affect blood vessels, smooth muscle, glands, and sensory nerves.
- Symptoms develop according to the tissues involved and the intensity of the reaction.
2. Important Mediators
| Mediator | Major effects |
|---|---|
| Histamine | Vasodilation, increased vascular permeability, itching, and smooth-muscle effects |
| Leukotrienes | Bronchoconstriction, mucus production, and increased vascular permeability |
| Prostaglandin D₂ | Contributes to bronchoconstriction and inflammatory responses |
| Cytokines, including IL-4, IL-5, and IL-13 | Support type 2 inflammation and recruitment or activation of relevant immune cells |
| Chemokines | Help recruit additional inflammatory cells to affected tissues |
3. Early-Phase and Late-Phase Reactions
Early-phase reaction: Usually begins within minutes of exposure. Histamine and other rapidly released mediators can cause itching, sneezing, swelling, mucus secretion, and bronchoconstriction.
Late-phase reaction: May develop several hours later. It involves recruited inflammatory cells, including eosinophils and T lymphocytes, and can prolong tissue inflammation. It is particularly relevant to persistent allergic airway inflammation.
4. Clinical Examples
- Allergic rhinitis: Sneezing, nasal itching, congestion, and watery nasal discharge after exposure to allergens.
- Urticaria: Raised, itchy wheals caused by transient changes in skin blood vessels.
- Food allergy: Reactions that may involve the skin, gastrointestinal tract, respiratory system, or circulation.
- Allergic asthma: In susceptible individuals, allergen-associated inflammation can contribute to airway narrowing and breathing difficulties.
- Anaphylaxis: A severe systemic allergic reaction that may cause airway compromise, breathing difficulties, and circulatory collapse.
Anaphylaxis is a medical emergency requiring prompt treatment; intramuscular epinephrine is the first-line treatment.
5. Diagnosis and Management Overview
Diagnosis depends on the clinical history and, when appropriate, skin-prick testing or measurement of allergen-specific IgE. A positive test indicates sensitization and must be interpreted alongside the person’s symptoms and exposure history.
Management varies by condition and severity. It may include allergen avoidance, appropriate antihistamines, intranasal corticosteroids for allergic rhinitis, or inhaled therapies for asthma. Allergen immunotherapy may be suitable for selected allergic conditions under specialist supervision. People at risk of anaphylaxis may require an epinephrine autoinjector and an emergency action plan.
Type I hypersensitivity is an IgE-mediated reaction in which allergen recognition activates mast cells and basophils, releasing mediators that produce immediate allergic symptoms and, in some cases, a later inflammatory response.
Type II Hypersensitivity — Antibody-Mediated Hypersensitivity
Type II hypersensitivity is an immune reaction in which immunoglobulin G (IgG) or immunoglobulin M (IgM) antibodies recognize antigens associated with cell surfaces or extracellular tissues. Antibody binding can cause cell destruction, inflammation, or abnormal changes in receptor function.
Unlike Type I hypersensitivity, which primarily involves IgE and mast-cell activation, Type II reactions usually involve IgG or IgM antibodies and their effects on specific cellular or tissue targets.
1. Mechanism of Type II Hypersensitivity
Type II hypersensitivity develops through several mechanisms.
A. Recognition of target antigens
IgG or IgM antibodies bind to antigens on cell membranes, extracellular structures, or receptors. These antigens may be naturally present in the body or introduced through certain medications or other exposures.
B. Complement activation
Antibodies bound to target cells can activate the classical complement pathway. Complement activation may promote inflammation, facilitate phagocytosis, or cause cell lysis in susceptible circumstances.
C. Opsonization and phagocytosis
Antibody-coated cells can be recognized by phagocytes through Fc receptors. C3b deposited through complement activation can also promote their recognition and removal. This mechanism is important in some forms of immune-mediated destruction of red blood cells.
D. Antibody-dependent cellular cytotoxicity
Natural killer (NK) cells and certain other immune cells can recognize antibody-coated targets through Fc receptors and contribute to cellular injury. This process is known as antibody-dependent cellular cytotoxicity (ADCC).
E. Alteration of receptor function
Some antibodies change the activity of cell-surface receptors without directly destroying the target cell. They may stimulate a receptor or block its normal ligand from binding.
2. Major Mechanisms and Examples
| Mechanism | What happens | Example |
|---|---|---|
| Complement-mediated or phagocytic cell destruction | Antibodies promote damage or removal of target cells | Autoimmune haemolytic anaemia |
| Antibody-dependent cellular cytotoxicity | Immune cells attack antibody-coated targets | Certain antibody-mediated cellular injuries |
| Inflammation against tissue antigens | Antibody binding recruits inflammatory mechanisms | Goodpasture disease |
| Receptor stimulation | Antibodies activate a receptor abnormally | Graves disease |
| Receptor blockade | Antibodies interfere with normal receptor function | Myasthenia gravis |
3. Important Clinical Examples
A. Autoimmune haemolytic anaemia
In autoimmune haemolytic anaemia, antibodies target red blood cell antigens. Depending on the antibody type and conditions, red blood cells may be destroyed through complement activation or removed by phagocytes, resulting in anaemia.
B. Haemolytic disease of the fetus and newborn
This condition can occur when maternal IgG antibodies cross the placenta and target fetal red blood cell antigens. Rh(D) incompatibility is a classic example. Maternal sensitization can often be prevented through appropriate anti-D immunoglobulin prophylaxis.
C. Goodpasture disease
In Goodpasture disease, autoantibodies target components of type IV collagen in basement membranes, particularly in the kidneys and lungs. This can produce rapidly progressive glomerulonephritis and pulmonary haemorrhage.
D. Graves disease
In Graves disease, antibodies stimulate the thyroid-stimulating hormone (TSH) receptor, causing excessive thyroid hormone production. This illustrates that antibody-mediated reactions can alter normal cell function rather than simply destroy cells.
E. Myasthenia gravis
In many cases of myasthenia gravis, antibodies interfere with communication between nerves and skeletal muscles, commonly by targeting acetylcholine receptors. Other antibody targets, including MuSK, may also be involved. The result is muscle weakness and fatigability.
4. Diagnosis and Clinical Significance
The diagnostic approach depends on the suspected disease and may include antibody testing, a complete blood count, complement measurements, tissue biopsy, or organ-specific investigations.
Type II hypersensitivity is clinically important because it can affect blood cells, kidneys, lungs, endocrine organs, and neuromuscular function. The treatment depends on the underlying condition and may involve immunosuppressive therapy, removal of harmful antibodies, or disease-specific treatment.
Type II hypersensitivity involves IgG or IgM antibodies directed against cell-associated or tissue antigens. These antibodies may trigger complement activation, phagocytosis, cellular cytotoxicity, or abnormal receptor activity.
Type III Hypersensitivity — Immune Complex-Mediated Hypersensitivity
Type III hypersensitivity is an immune reaction caused by the formation and deposition of antigen–antibody complexes in tissues or blood vessel walls. These complexes activate the complement system and recruit inflammatory cells, particularly neutrophils, resulting in tissue inflammation and damage.
Unlike Type II hypersensitivity, in which antibodies bind directly to antigens on cell surfaces or in tissues, Type III hypersensitivity primarily involves circulating immune complexes that deposit in tissues.
Mechanism of Type III Hypersensitivity
The reaction develops through the following steps:
- Immune complex formation: Soluble antigens bind to specific antibodies, usually IgG and sometimes IgM, forming antigen–antibody complexes.
- Deposition of immune complexes: When complexes are not efficiently cleared, they may deposit in tissues such as blood vessel walls, renal glomeruli, joints, and skin.
- Complement activation: Deposited complexes activate the classical complement pathway, generating inflammatory mediators such as C3a and C5a.
- Recruitment of inflammatory cells: C5a attracts neutrophils to the affected site.
- Tissue damage: Neutrophils release lysosomal enzymes and reactive oxygen species, damaging nearby tissues and producing inflammation.
Examples of Type III Hypersensitivity
1. Systemic lupus erythematosus (SLE)
SLE is an autoimmune disease in which autoantibodies form immune complexes with self-antigens. These complexes may deposit in the kidneys, skin, joints, blood vessels, and other tissues. Immune complex deposition contributes to lupus nephritis and other inflammatory manifestations.
2. Serum sickness
Serum sickness is a systemic reaction that may occur after exposure to certain foreign proteins or medications. Immune complexes form and deposit in tissues, causing symptoms such as fever, rash, joint pain, and sometimes lymph node enlargement.
3. Arthus reaction
The Arthus reaction is a localized Type III hypersensitivity reaction that occurs when antigen encounters sufficient pre-existing antibodies in a tissue. Immune complexes form locally, causing complement activation, inflammation, swelling, and sometimes tissue necrosis.
4. Post-streptococcal glomerulonephritis
This kidney disorder can develop after certain streptococcal infections. Immune mechanisms, including immune complex deposition or formation within the glomeruli, contribute to inflammation and impaired kidney function.
5. Some forms of vasculitis
Immune complex deposition in blood vessel walls can cause inflammation and damage to vessels. The clinical effects depend on the size and location of the affected vessels and the underlying disease.
Clinical Features
The signs and symptoms depend on where immune complexes accumulate and the intensity of the inflammatory response. Common features may include:
- Fever and malaise
- Skin rash
- Joint pain and swelling
- Inflammation of blood vessels
- Protein or blood in the urine when the kidneys are affected
- Tissue swelling and local inflammation
Diagnosis: Diagnosis depends on the suspected disease and may include:
- Clinical history and physical examination
- Complement measurements, including C3 and C4
- Tests for disease-specific antibodies, such as antinuclear antibodies in suspected SLE
- Urinalysis and kidney function tests when renal involvement is suspected
- Tissue biopsy in selected cases to identify inflammation or immune deposits
Low serum complement levels may support complement consumption in some immune complex-mediated diseases, but normal levels do not exclude the diagnosis.
Management Overview
Treatment depends on the underlying cause, the organs involved, and disease severity. It may include removing or treating the triggering cause, anti-inflammatory medication, or immunosuppressive treatment for selected autoimmune diseases. Serious kidney or blood vessel involvement requires specialist assessment.
Key Distinction
The central feature of Type III hypersensitivity is immune complex deposition followed by complement activation and inflammation. It is classically associated with conditions such as serum sickness, the Arthus reaction, and immune complex-mediated manifestations of SLE.
Type IV Hypersensitivity — Delayed-Type Hypersensitivity
Type IV hypersensitivity is a cell-mediated immune reaction caused primarily by sensitized T lymphocytes rather than antibodies. It is also known as delayed-type hypersensitivity (DTH) because the reaction typically develops over 24–72 hours after exposure to the relevant antigen, although timing varies with the condition.
In this type of hypersensitivity, antigen-specific T cells release cytokines, activate macrophages, or directly kill target cells. These processes produce inflammation and tissue injury. Unlike Types I, II, and III, Type IV hypersensitivity does not depend on antibodies as the primary effector mechanism.
Mechanism of Type IV Hypersensitivity
The reaction generally involves two stages:
1. Sensitization phase
- An antigen enters the body and is taken up by antigen-presenting cells, such as dendritic cells.
- These cells process the antigen and present its peptides to T lymphocytes.
- Antigen-specific T cells become activated and expand.
- Memory T cells develop and can respond more rapidly during subsequent exposure.
2. Effector phase
- On re-exposure, memory T cells recognize the antigen.
- CD4+ T cells release cytokines that recruit and activate macrophages and other inflammatory cells.
- CD8+ cytotoxic T cells may directly destroy antigen-bearing target cells.
- The resulting cellular response causes inflammation and, in some conditions, tissue damage.
Major Mechanisms of Type IV Hypersensitivity
1. Th1 cell-mediated reactions
Th1 cells release cytokines, including interferon-gamma (IFN-γ), which activates macrophages. Activated macrophages contribute to inflammation and tissue injury.
Example: The tuberculin skin test.
2. Th17 cell-mediated reactions
Th17 cells produce cytokines such as IL-17, which help recruit neutrophils and promote inflammation. These responses may contribute to certain inflammatory and autoimmune diseases.
3. CD8+ cytotoxic T-cell reactions
CD8+ T cells recognize antigen-derived peptides presented by MHC class I molecules and may kill target cells through perforin–granzyme pathways or other cytotoxic mechanisms.
Example: T-cell-mediated injury to infected cells and some forms of drug-induced skin reactions.
Examples of Type IV Hypersensitivity
1. Contact dermatitis
Contact dermatitis can occur after skin exposure to substances such as nickel, fragrances, or certain chemicals. Antigen-specific T cells trigger inflammation, producing redness, itching, swelling, and sometimes small blisters. The reaction often becomes evident one to three days after exposure.
2. Tuberculin skin test
The tuberculin skin test evaluates a delayed cellular immune response to purified protein derivative (PPD). In a previously sensitized person, antigen-specific T cells produce localized induration, usually assessed 48–72 hours after administration. A positive result indicates immune sensitization but does not by itself establish active tuberculosis.
3. Granulomatous inflammation in tuberculosis
In tuberculosis, T-cell responses activate macrophages and can contribute to granuloma formation. Granulomas help contain infection but may also cause tissue damage. The overall disease process is complex and involves both immune-mediated injury and the effects of the pathogen.
4. Type 1 diabetes mellitus
In Type 1 diabetes, autoreactive T lymphocytes contribute to the destruction of insulin-producing pancreatic beta cells. This is an example of T-cell-mediated autoimmune injury, although the disease involves multiple immune mechanisms.
5. Some drug reactions
Certain delayed drug hypersensitivity reactions are mediated by antigen-specific T cells. Depending on the reaction, symptoms may range from a skin rash to severe systemic illness. Serious reactions require prompt medical evaluation.
Clinical Features: The features vary according to the affected tissue and the responsible T-cell response. They may include:
- Redness and swelling at the affected site
- Itching or skin rash
- Localized induration
- Inflammatory cell infiltration
- Tissue injury in affected organs
- Granuloma formation in some persistent infections
Diagnosis: Diagnosis depends on the suspected condition and may include:
- Clinical history and physical examination
- Patch testing for suspected allergic contact dermatitis
- Tuberculin skin testing or interferon-gamma release assays when evaluating tuberculosis infection
- Blood tests, imaging, or tissue biopsy when indicated
- Specialist evaluation for suspected severe drug reactions or autoimmune disease
No single test diagnoses all forms of Type IV hypersensitivity.
Management Overview
Management depends on the cause. Avoidance of the responsible contact allergen may help prevent contact dermatitis. Topical corticosteroids may be used for selected inflammatory skin reactions under appropriate medical guidance. Infections require disease-specific treatment, while autoimmune conditions may need specialist-directed immunomodulatory therapy.
Key Distinction
Type IV hypersensitivity is primarily T-cell mediated and antibody independent. It commonly develops more slowly than immediate IgE-mediated reactions and is associated with contact dermatitis, delayed skin-test responses, and several forms of T-cell-mediated tissue injury.
Comparison of the Four Types of Hypersensitivity
The four types of hypersensitivity reactions are classified according to their immune mechanisms, principal effector components, and patterns of tissue injury. This classification, developed by Gell and Coombs, is widely used in immunology to understand allergic reactions, antibody-mediated diseases, immune complex disorders, and T-cell-mediated inflammation.
| Feature | Type I | Type II | Type III | Type IV |
|---|---|---|---|---|
| Common name | Immediate | Antibody-mediated | Immune complex-mediated | Delayed-type |
| Main immune component | IgE, mast cells, basophils | IgG or IgM antibodies | Antigen–antibody complexes | T lymphocytes |
| Main mechanism | Mast-cell activation and mediator release | Antibodies bind cell-surface or tissue antigens | Immune complexes deposit in tissues and activate inflammation | T cells activate inflammatory cells or kill target cells |
| Complement involvement | Not the primary mechanism | May be important | Frequently important | Not the primary defining mechanism |
| Typical onset | Minutes; late-phase inflammation may follow | Variable | Often hours, depending on the condition | Commonly 24–72 hours in classic delayed reactions |
| Common sites affected | Skin, respiratory tract, gastrointestinal tract | Blood cells, basement membranes, receptors and tissues | Blood vessels, kidneys, joints and skin | Skin and other tissues containing antigen-specific T-cell responses |
| Examples | Anaphylaxis, allergic rhinitis, some forms of asthma | Autoimmune haemolytic anaemia, Graves disease | Serum sickness, Arthus reaction, some manifestations of SLE | Contact dermatitis, tuberculin skin-test reaction |
| Main injury mechanism | Histamine and other inflammatory mediators | Cell destruction, inflammation or altered receptor function | Inflammation caused by deposited immune complexes | Cytokine-mediated inflammation or cytotoxic T-cell activity |
Note: These are the classical mechanisms. Some diseases involve overlapping immune pathways, and their timing can vary.
Key Differences Explained
1. Role of antibodies
- Type I: IgE binds to mast cells and basophils; allergen-mediated cross-linking triggers mediator release.
- Type II: IgG or IgM binds directly to antigens on cell surfaces or within tissues.
- Type III: Antibodies form complexes with soluble antigens, and the complexes may deposit in tissues.
- Type IV: T lymphocytes are the primary effectors; antibodies are not the defining mechanism.
2. Role of immune cells
Type I reactions prominently involve mast cells and basophils. Type II reactions may involve phagocytes, natural killer cells, or complement-mediated injury. Type III reactions commonly recruit neutrophils, whereas Type IV reactions involve antigen-specific T cells and, in many cases, macrophages.
3. Time of onset
Type I reactions often begin within minutes. Types II and III have variable onset depending on the disease and the mechanism involved. Classic Type IV delayed reactions commonly become apparent after 24–72 hours.
4. Nature of tissue damage
Type I reactions primarily cause mediator-driven inflammation and smooth-muscle contraction. Type II reactions can destroy cells or interfere with receptor function. Type III reactions cause inflammation following immune complex deposition. Type IV reactions produce cellular inflammation or direct T-cell-mediated injury.
- Type I — Immediate: IgE and mast cells.
- Type II — Target: Antibodies attack cell-surface or tissue antigens.
- Type III — Complexes: Immune complexes deposit in tissues.
- Type IV — T cells: Cell-mediated and often delayed.
These memory cues are useful for examinations, but they should not replace understanding the underlying mechanisms.
Biological and Clinical Significance of Hypersensitivity
Hypersensitivity reactions are important in immunology because they demonstrate how immune mechanisms that normally protect the body can also cause tissue injury. Although immune responses help eliminate pathogens and harmful substances, inappropriate, excessive, or misdirected responses may contribute to allergic diseases, autoimmune disorders, and inflammatory tissue damage.
Understanding hypersensitivity mechanisms helps explain disease development, identify diagnostic approaches, and guide appropriate treatment.
1. Role in Allergic Diseases
Type I hypersensitivity is responsible for many common allergic conditions. In susceptible individuals, allergens stimulate IgE production and activate mast cells and basophils.
Examples include:
- Allergic rhinitis
- Allergic conjunctivitis
- Urticaria (hives)
- Some forms of allergic asthma
- Food allergies
- Anaphylaxis
These conditions vary in severity. Anaphylaxis is a medical emergency that requires prompt intramuscular epinephrine and emergency medical care.
2. Role in Autoimmune Diseases
In autoimmune diseases, immune responses are directed against the body’s own components. Some autoimmune conditions involve mechanisms resembling Type II or Type III hypersensitivity, while T-cell-mediated mechanisms contribute to others.
Examples include:
- Autoimmune haemolytic anaemia: Antibodies target red blood cells, promoting their destruction.
- Graves disease: Autoantibodies stimulate thyroid-stimulating hormone receptors.
- Systemic lupus erythematosus (SLE): Immune complexes contribute to inflammation and organ injury.
- Type 1 diabetes mellitus: Autoreactive T cells contribute to pancreatic beta-cell destruction.
These examples illustrate how different immune mechanisms can produce distinct clinical outcomes.
3. Role in Tissue and Organ Damage
Hypersensitivity reactions may affect specific tissues or multiple organs. Antibody-mediated injury can affect blood cells or basement membranes, immune complexes can damage blood vessels and kidneys, and T-cell-mediated reactions can cause skin inflammation or injury to other tissues.
The severity depends on the mechanism, the amount and persistence of antigen exposure, the affected organ, and the intensity of the immune response.
4. Role in Diagnosis
Identifying the underlying immune mechanism helps clinicians select appropriate tests.
Examples include:
- Skin-prick testing or allergen-specific IgE testing: Used in evaluating suspected immediate allergic reactions.
- Direct antiglobulin test (DAT): Helps investigate antibody- or complement-associated red blood cell destruction.
- Autoantibody testing and complement measurements: May support the evaluation of selected autoimmune or immune complex-mediated diseases.
- Patch testing: Helps identify allergic contact dermatitis.
- Tuberculin skin testing: Evaluates a delayed cellular immune response to tuberculosis-related antigens, although it cannot by itself distinguish active disease from infection without active disease.
Test results must be interpreted alongside symptoms, clinical history, and other findings.
5. Role in Treatment and Prevention
Understanding the mechanism helps guide management.
- Allergic diseases: Avoidance of relevant allergens, appropriate antihistamines, inhaled treatments, or other condition-specific therapies may be used.
- Anaphylaxis: Intramuscular epinephrine is the first-line emergency treatment.
- Antibody-mediated diseases: Treatment may involve disease-specific medication, immunomodulation, or supportive care.
- Immune complex-mediated diseases: Management focuses on the underlying cause and the severity of organ involvement.
- T-cell-mediated conditions: Treatment may include allergen avoidance, topical anti-inflammatory therapy, infection-specific treatment, or specialist-directed immunomodulation.
Not all hypersensitivity reactions can be prevented, but recognising triggers and treating underlying conditions can reduce risk and complications.
6. Importance in Immunological Research
Hypersensitivity reactions help researchers understand immune tolerance, antibody function, cytokine signalling, complement activation, mast-cell biology, and T-cell responses. This knowledge has contributed to targeted treatments for allergic, autoimmune, and inflammatory diseases.
Hypersensitivity is clinically significant because it links specific immune mechanisms to disease symptoms, tissue injury, diagnostic findings, and treatment decisions. Correctly identifying the mechanism is essential for effective clinical management.
Diagnosis and Management of Hypersensitivity
The diagnosis and management of hypersensitivity reactions depend on the underlying immune mechanism, the organs affected, the severity of symptoms, and the suspected trigger. A detailed clinical history and physical examination are important starting points, followed by tests selected according to the suspected condition.
There is no single diagnostic test or treatment suitable for all four types.
Diagnostic Approaches
1. Clinical history and physical examination
The clinician evaluates the timing of symptoms, possible allergen or drug exposure, previous reactions, family history, and affected organs. The onset and pattern of symptoms can help distinguish immediate allergic reactions from delayed cellular reactions.
2. Skin-prick testing
Skin-prick testing may help identify immediate IgE-mediated sensitisation to specific allergens. It is commonly used when evaluating allergic rhinitis, selected food allergies, and other suspected immediate allergic reactions. Results must be interpreted alongside the patient’s clinical history because a positive result indicates sensitisation, not necessarily clinical allergy.
3. Allergen-specific IgE testing
Blood tests can measure IgE directed against particular allergens. These tests may be useful when skin testing is unsuitable or when additional information is required. Total IgE alone cannot reliably identify the cause of an allergy.
4. Antibody and complement testing
Selected blood tests can support the evaluation of antibody-mediated or immune complex-associated diseases. Examples include the direct antiglobulin test for suspected immune-mediated red blood cell destruction, disease-specific autoantibodies, and complement measurements such as C3 and C4.
5. Patch testing
Patch testing is used to investigate allergic contact dermatitis. Suspected allergens are applied to the skin under controlled conditions, and the skin is assessed at specified intervals for a delayed reaction.
6. Biopsy and additional investigations
In selected cases, tissue biopsy may help identify immune deposits, inflammatory changes, or cellular injury. Blood counts, kidney function tests, urinalysis, imaging, and other investigations may be required according to the suspected disease.
Management Principles
Management focuses on identifying and reducing exposure to the trigger when possible, controlling inflammation, preventing further reactions, and treating any underlying disease.
- Trigger avoidance: Avoidance of a confirmed allergen, contact substance, or causative medication can reduce recurrence.
- Medication: Depending on the condition, treatment may include antihistamines, corticosteroids, inhaled therapies, or other disease-specific medications.
- Allergen immunotherapy: Selected patients with particular allergic conditions may benefit from specialist-supervised allergen immunotherapy.
- Immune-directed treatment: Some autoimmune or immune complex-mediated disorders require immunomodulatory or immunosuppressive treatment under specialist supervision.
- Follow-up: Monitoring helps assess treatment response, detect complications, and revise the management plan when necessary.
Emergency Management of Anaphylaxis
Anaphylaxis is a potentially life-threatening systemic allergic reaction. Intramuscular epinephrine (adrenaline) is the first-line treatment. Emergency medical services should be contacted immediately. Antihistamines do not replace epinephrine, and treatment should not be delayed while waiting for other medications to work.
Prevention and Patient Education
Prevention depends on the cause. Patients may benefit from learning how to recognise early symptoms, avoid known triggers, follow prescribed treatment plans, and use an emergency action plan when indicated. People at risk of anaphylaxis should discuss whether they need an epinephrine auto-injector and receive training in its use.
Important: Diagnosis and treatment should be individualised by a qualified healthcare professional. Severe breathing difficulty, throat or tongue swelling, collapse, or signs of anaphylaxis require immediate emergency care.
Frequently Asked Questions (FAQs) About Hypersensitivity
What is hypersensitivity?
What are the four types of hypersensitivity?
Which type of hypersensitivity is mediated by IgE?
What is the difference between Type II and Type III hypersensitivity?
Why is Type IV hypersensitivity called delayed-type hypersensitivity?
What are examples of Type I hypersensitivity?
What are examples of Type II hypersensitivity?
What are examples of Type III hypersensitivity?
What are examples of Type IV hypersensitivity?
Which type of hypersensitivity involves the complement system?
How is hypersensitivity diagnosed?
Is hypersensitivity always an allergy?
Key Takeaways
- Hypersensitivity refers to an immune response that causes tissue injury or disease.
- The classical Gell and Coombs classification recognises four major types of hypersensitivity reactions.
- Type I: Immediate reaction mediated by IgE, mast cells, and basophils.
- Type II: Antibody-mediated reaction involving IgG or IgM against cell-surface or tissue antigens.
- Type III: Immune complex-mediated reaction involving deposition of antigen–antibody complexes and inflammation.
- Type IV: T-cell-mediated reaction, commonly called delayed-type hypersensitivity.
- Type I reactions can produce histamine release, bronchoconstriction, swelling, and anaphylaxis.
- Type II reactions may cause cell destruction, inflammation, or altered receptor function.
- Type III reactions commonly involve complement activation and neutrophil recruitment.
- Type IV reactions involve cytokine-mediated inflammation, macrophage activation, or cytotoxic T-cell activity.
- Examples of Type I include allergic rhinitis and anaphylaxis.
- Examples of Type II include autoimmune haemolytic anaemia and Graves disease.
- Examples of Type III include serum sickness and the Arthus reaction.
- Examples of Type IV include allergic contact dermatitis and the tuberculin skin-test reaction.
- The classical onset of Type I is within minutes, while Type IV reactions commonly develop over 24–72 hours.
- Diagnosis and treatment depend on the suspected mechanism, clinical presentation, and affected organs.
- Anaphylaxis is a medical emergency; intramuscular epinephrine is the first-line treatment.
Conclusion
Hypersensitivity reactions are important immunological responses in which immune mechanisms cause tissue injury or disease. The classical Gell and Coombs classification divides these reactions into four types based on their underlying mechanisms: IgE-mediated immediate reactions, antibody-mediated tissue injury, immune complex-mediated inflammation, and T-cell-mediated delayed reactions.
Each type has distinct mediators, mechanisms, clinical manifestations, and diagnostic approaches. Understanding these differences helps explain allergic diseases, autoimmune disorders, inflammatory tissue damage, and several clinically significant immune reactions.
Accurate identification of the underlying mechanism is essential for diagnosis, prevention, and appropriate treatment. Studying the four types of hypersensitivity therefore provides a fundamental understanding of both protective immunity and immune-mediated disease.
References
- Abbas, A. K., Lichtman, A. H., and Pillai, S. Cellular and Molecular Immunology. Elsevier. A standard textbook covering immune mechanisms, allergic reactions, autoimmunity, and immunopathology.
https://www.elsevier.com/ - Murphy, K., and Weaver, C. Janeway’s Immunobiology. W. W. Norton & Company. A comprehensive reference for antibody-mediated immunity, T-cell responses, and immune-mediated disease.
https://wwnorton.com/ - Kumar, V., Abbas, A. K., and Aster, J. C. Robbins & Cotran Pathologic Basis of Disease. Elsevier. Covers the pathological mechanisms of immune-mediated tissue injury and autoimmune disease.
https://www.elsevier.com/ - Gell, P. G. H., and Coombs, R. R. A. (eds.). (1963). Clinical Aspects of Immunology. Blackwell Scientific Publications. A foundational reference associated with the classical classification of hypersensitivity reactions.
- Merck Manual Professional Edition. Overview of Allergic Reactions.
https://www.merckmanuals.com/professional/immunology-allergic-disorders/allergic-reactions-and-other-hypersensitivity-disorders/overview-of-allergic-reactions - Merck Manual Professional Edition. Overview of the Immune System.
https://www.merckmanuals.com/professional/immunology-allergic-disorders/biology-of-the-immune-system/overview-of-the-immune-system - National Institute of Allergy and Infectious Diseases (NIAID). Food Allergy.
https://www.niaid.nih.gov/diseases-conditions/food-allergy - MedlinePlus. Allergy. U.S. National Library of Medicine.
https://medlineplus.gov/allergy.html - StatPearls. Type I Hypersensitivity Reaction. National Center for Biotechnology Information, Bookshelf.
https://www.ncbi.nlm.nih.gov/books/ - National Center for Biotechnology Information (NCBI), PubMed. Research literature on hypersensitivity mechanisms and immune-mediated diseases.
https://pubmed.ncbi.nlm.nih.gov/?term=hypersensitivity+reactions+immunology
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