### Date : 2024-06-23 21:02 ### Topic : Hypersensitivity Reaction #medicine #immunology #hypersensitivityreaction #allergy ---- Hypersensitivity reactions are immune responses that are exaggerated or inappropriate against an antigen or allergen. These reactions can cause tissue damage and disease. They are classified into four types: Type I, II, III, and IV, each with distinct mechanisms and clinical manifestations. ### Type I Hypersensitivity (Immediate) - **Mechanism**: IgE-mediated response - **Antigens**: Typically environmental allergens (e.g., pollen, dust mites, pet dander) - **Process**: 1. **Sensitization Phase**: Initial exposure to the allergen leads to the production of IgE antibodies by B cells. These IgE antibodies bind to the surface of mast cells and basophils. 2. **Effector Phase**: Upon re-exposure to the same allergen, it binds to the IgE on mast cells and basophils, causing these cells to degranulate and release histamine and other inflammatory mediators. - **Clinical Manifestations**: Symptoms can range from mild (allergic rhinitis, urticaria) to severe (anaphylaxis). - **Examples**: - Hay fever (allergic rhinitis) - Asthma - Anaphylaxis (severe systemic reaction that can be life-threatening) ### Type II Hypersensitivity (Cytotoxic) - **Mechanism**: IgG or IgM antibodies directed against cell surface antigens or extracellular matrix components. - **Process**: 1. **Antibody Binding**: Antibodies bind to antigens on the surface of target cells. 2. **Cell Destruction**: This binding can lead to cell destruction through: - **Complement Activation**: Formation of the membrane attack complex (MAC) leading to cell lysis. - **Opsonization and Phagocytosis**: Phagocytes recognize and ingest the antibody-coated cells. - **Antibody-Dependent Cellular Cytotoxicity (ADCC)**: Natural killer (NK) cells induce apoptosis in the target cells. - **Clinical Manifestations**: Tissue-specific reactions, often involving blood cells. - **Examples**: - Hemolytic anemia - Goodpasture's syndrome - Myasthenia gravis ### Type III Hypersensitivity (Immune Complex-Mediated) - **Mechanism**: Immune complexes (antigen-antibody complexes) deposit in tissues, leading to inflammation and tissue damage. - **Process**: 1. **Formation of Immune Complexes**: Antibodies (usually IgG) bind to soluble antigens, forming immune complexes. 2. **Deposition**: These complexes circulate and deposit in various tissues (e.g., kidneys, joints, blood vessels). 3. **Inflammation**: Complement activation and the recruitment of inflammatory cells (neutrophils) lead to tissue damage. - **Clinical Manifestations**: Systemic or localized inflammation and tissue damage. - **Examples**: - Systemic lupus erythematosus (SLE) - Post-streptococcal glomerulonephritis - Serum sickness ### Type IV Hypersensitivity (Delayed-Type) - **Mechanism**: T-cell mediated immune response - **Process**: 1. **Sensitization Phase**: Initial exposure to an antigen leads to the activation of T-helper cells (Th1 or Th17). 2. **Effector Phase**: Upon re-exposure, sensitized T-cells release cytokines that recruit and activate macrophages and other inflammatory cells. - **Clinical Manifestations**: Delayed inflammatory response, typically 48-72 hours after exposure. - **Examples**: - Contact dermatitis (e.g., poison ivy) - Tuberculin skin test (Mantoux test) - Chronic transplant rejection ### Summary Hypersensitivity reactions involve different immune mechanisms and lead to various clinical manifestations. Understanding the specific type of hypersensitivity is crucial for diagnosing and treating related conditions effectively. | Type | Mediator | Mechanism | Examples | |-------|----------|-----------|----------| | I | IgE | Mast cell degranulation | Anaphylaxis, hay fever | | II | IgG, IgM | Antibody-mediated cell destruction | Hemolytic anemia, Goodpasture's syndrome | | III | Immune complexes | Immune complex deposition | SLE, serum sickness | | IV | T-cells | T-cell mediated inflammation | Contact dermatitis, TB skin test |