Hypersensitivity Reactions

Overview

Hypersensitivity reactions are immune responses that are exaggerated or inappropriate against an antigen or allergen. These reactions are categorized into four types, each with distinct mechanisms and effects on the body.

Type 1 Hypersensitivity

• Antigen: External allergens (e.g., pollen, certain foods)
• Mechanism:
  • Antigen binds to IgE antibodies on mast cells or basophils.
  • Triggers intracellular reactions leading to histamine release.
• Effects:
  • Causes allergic reactions of varying severity.

Type 2 Hypersensitivity

• Antigen: Molecules on the surface of patient cells.
• Mechanism:
  • IgG or IgM antibodies bind to cell-surface molecules.
  • Activates complement system, natural killer cells, and macrophages.
• Effects:
  • Results in damage to affected tissues.

Type 3 Hypersensitivity

• Antigen: Small antigen-antibody complexes.
• Mechanism:
  • Complexes accumulate in tissues (joints, glomeruli, blood vessels).
  • Triggers complement system to attack tissues.

Type 4 Hypersensitivity

• Antigen: Intracellular antigens (e.g., fungi, Mycobacterium tuberculosis).
• Mechanism:
  • T-helper cells (particularly CD4 cells) recognize antigens.
  • CD4 cells release cytokines, activating phagocytes.
• Effects:
  • Phagocytes attack and engulf affected cells.

Summary

• Each type of hypersensitivity reaction involves different immune components and mechanisms.
• Types 1, 2, and 3 are antibody-mediated, while type 4 is cell-mediated.
• These reactions result in various immune responses and tissue damages.