BIO2215 - Unit 10 - Type I Hypersensitivity

Introduction to Allergic Reactions

• Some people experience allergic reactions or hypersensitivity when exposed to various substances such as:
  • Dust
  • Pollens
  • Animal dander
  • Penicillin
• This hypersensitivity is mediated primarily by Immunoglobulin E (IgE).

Sensitization Process

• Contact with Antigen:
  • Sensitization begins when an antigen comes into contact with the body.
• Uptake and Processing:
  • Antigen is captured and processed by antigen-presenting cells (APCs).
• Presentation to T helper Cells:
  • The processed antigen is presented on Class II Major Histocompatibility Complex (MHC II) molecules to T helper cells.
• B Cells and IgE Production:
  • Tissues beneath the mucous membranes are rich in B cells dedicated to IgE production.
  • Individuals prone to allergies tend to have higher numbers of IgE-producing cells.

Role of Cytokines

• Activation of B Cells:
  • T helper cells produce cytokines, which promote the proliferation and differentiation of B cells into IgE-producing plasma cells.
• IgE Attachment:
  • As IgE is generated, the antibodies attach to receptors on nearby mast cells via their constant regions.
  • Mast cells contain granules loaded with chemicals crucial for inducing hypersensitivity responses.
  • IgE molecules can persist on mast cells for several weeks, leading to sensitization to the specific antigen.

Secondary Exposure and Reaction

• Antigen Rebinding:
  • Upon re-exposure to the same antigen, it binds to the IgE antibodies present on mast cells.
• Requirement for Reaction:
  • Two cell-bound IgE molecules must cross-link with the specific antigen to initiate a response.
• Mast Cell Degranulation:
  • This cross-linking triggers the mast cell to release histamine and various mediators of the inflammatory response within seconds.
  • The release of these substances leads to a spectrum of allergic symptoms, demonstrating the body’s hypersensitivity to the antigen.