Lecture: Types of Hypersensitivity Reactions

Overview

• Four types: Type 1, Type 2, Type 3, and Type 4
• Classification based on reaction time:
  • Type 1: Fastest
  • Type 4: Slowest
• Recent video covered the complement system and its three pathways

Type 1 Hypersensitivity: Immediate Reaction

• Characteristics:
  • Immediate, anaphylactic, involves IgE and mast cells
  • Mast cells degranulate, releasing histamine
  • Symptoms: bronchoconstriction, vasodilation, increased capillary permeability
• Phases:
  • Early Phase: Release of pre-formed mediators (histamine, serotonin)
  • Late Phase: Production of prostaglandins, leukotrienes, and platelet-activating factors
• Examples: Allergic asthma, bee sting reactions

Type 2 Hypersensitivity: Cytotoxic Reaction

• Characteristics:
  • Antigen-antibody complexes on cell surfaces
  • Involves IgG or IgM
  • Can cause inflammation, cell dysfunction, or stimulate target organs
• Examples:
  • Inhibition: Myasthenia gravis, Goodpasture syndrome
  • Stimulation: Graves disease
  • Rejection phenomena: Hyperacute transplant rejection

Type 3 Hypersensitivity: Immune Complex-Mediated Reaction

• Characteristics:
  • Antigen-antibody complexes float freely in plasma
  • Can cause deposition in tissues (vasculitis, arthritis, nephritis)
• Types:
  • Generalized (e.g., serum sickness): Involves formation of antibodies against foreign antibodies
  • Localized (e.g., Arthus reaction): Localized reaction due to pre-sensitization
• Examples: Lupus, rheumatoid arthritis

Type 4 Hypersensitivity: Delayed, Cell-Mediated Reaction

• Characteristics:
  • Does not involve antibodies; cell-mediated immunity
  • Involves T lymphocytes
  • Can cause direct cell destruction or granuloma formation
• Examples:
  • Tuberculin skin test, poison ivy, contact dermatitis
  • Granulomas in tuberculosis, sarcoidosis

Detailed Mechanisms

• First Exposure: Antigen presenting cells process antigen and present to naive B and T cells
• Second Exposure: Involves re-exposure to antigen, leading to a more intense immune response
• Immunological Processes:
  • Helper T cells (Th1 and Th2) role in hypersensitivities
  • Role of cytokines such as interleukin-4, interleukin-5

Clinical Management

• Asthma management:
  • Use of leukotriene inhibitors, mast cell stabilizers, inhaled corticosteroids
  • Avoidance of beta-blockers and aspirin in asthmatic patients

Additional Notes

• Historical classifications (e.g., Type 5) and their obsolescence
• Role of monoclonal antibodies in treating hypersensitivity-related disorders
• Granuloma formation: Involves CD4 T cells, macrophages, and fibroblasts, particularly in high-virulence infections like tuberculosis

Conclusion

• Understanding hypersensitivity reactions aids in diagnosis and treatment of various immune disorders
• Future topics will delve deeper into specific immune pathways and their clinical relevance