Allergic Reactions and Hypersensitivity

Introduction

• Some individuals develop allergic reactions or hypersensitivity to substances such as:
  • Dust
  • Pollens
  • Animal dander
  • Penicillin
• Hypersensitivity is mediated by Immunoglobulin E (IgE).

Sensitization Process

• Contact with Antigen:

  • Antigen exposure to the body is the first step.
  • Antigen is processed by antigen-presenting cells.
  • Presented on a class 2 MHC to T helper cells.

• B-Cells and IgE Production:

  • Tissues under mucous membranes are rich in B-cells committed to IgE production.
  • IgE-producing cells are more abundant in those susceptible to allergies.
  • T helper cells produce cytokines that stimulate B cells.
  • B cells proliferate and differentiate into IgE-producing plasma cells.

IgE Attachment

• IgE molecules attach to receptors on nearby mast cells via their constant regions.
• Mast cells contain granules with chemicals that induce a hypersensitivity response.
• IgE molecules can survive for many weeks on mast cells, indicating sensitization.

Hypersensitivity Reaction

• Secondary Exposure to Antigen:

  • Antigen binds to IgE antibodies on mast cells, triggering a response.
  • At least two cell-bound IgE molecules must react with a specific antigen.

• Response Mechanism:

  • Within seconds of the reaction, mast cells release histamine and other inflammatory mediators from granules.
  • This release triggers a variety of symptoms associated with allergic reactions.

Key Points

• IgE sensitization occurs when an antigen is processed and presented to T helper cells.
• B cells are stimulated to produce IgE, which binds to mast cells.
• On secondary exposure, a rapid inflammatory response occurs, leading to allergic symptoms.