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Understanding Antigens in Immunology

Apr 14, 2025

Immunology: Antigens

Introduction

  • Discussion on antigens, focusing on:
    • Definition
    • Properties
    • Structure
    • Types
    • Antigen-antibody complex
    • Examples
    • Applications

Antigen Definition

  • Molecules or structures not recognized by the body.
  • Often cause antibody formation due to immune response.
  • Can be anything foreign to the body.
  • Immunogens are antigens that cause reactions.
  • Antigenic determinants are specific areas inducing immune response.
  • Can be bacteria, viruses, chemicals, pollen.
  • Proteins, peptides, polysaccharides, lipids, and nucleic acids can be antigens.

Properties of Antigens

  1. Foreign Nature

    • External to the host's body.
    • More foreign = higher immunogenicity.
    • Some host proteins can also trigger immune response.
  2. Chemical Nature

    • Proteins and polysaccharides are potent antigens.
    • Immunogenic portion must have specific amino acids and charged groups.
  3. Molecular Size

    • Must be at least 50,000 Dalton to be immunogenic.
    • Haptens are partial antigens with low molecular weight.
  4. Molecular Rigidity and Complexity

    • Rigid molecules may produce specific antibodies.
    • Complexity important for immunogenicity.
  5. Antigenic Determinants and Cross Reactivity

    • Parts of an antigen involved in antibody response.
    • Cross-reactivity is the interaction of an antibody with different antigens.

Antigen Structure

  • Ability to attach to antibody binding site.
  • Unique molecular structure on surface.
  • Polysaccharides or proteins typically.
  • Structural elements like epitopes determine interactions.

Types of Antigens

  1. Based on Origin

    • Exogenous Antigens: From outside host, ingested or inhaled.
    • Endogenous Antigens: Develop naturally within host due to infection.
    • Autoantigens: Cause autoimmune diseases.
    • Tumor Antigens: Present on tumor cells.
    • Native Antigens: Unaltered by antigen-presenting cells.
  2. Based on Immune Response

    • Complete Antigens (Immunogens): Can elicit immune response alone.
    • Incomplete Antigens (Haptens): Require carrier molecules to elicit response.

Antigen Processing and Presentation

  • Antigen-presenting cells break antigens into peptides.
  • Endogenous Pathway: Intracellular proteins presented by MHC class 1.
  • Exogenous Pathway: External proteins presented by MHC class 2.
  • Cross Presentation: Antigens redirected to MHC class 1.

Antigen-Antibody Complex

  • Combination known as immunogenic complex.
  • Specific interaction based on amino acid sequences.
  • Complexes can lead to various reactions.

Antigen Examples

  1. Blood Group Antigens

    • Proteins/sugars on red blood cells.
    • Determine blood type.
  2. Bacterial Capsule

    • Polysaccharide layer causing immune response.
    • Involved in pathogenicity.

Applications of Antigens

  • Diagnostic tools for identifying bacterial species.
  • Used in forensic applications.
  • Employed in vaccines as inactivated enzymes.

Conclusion

  • Antigens have crucial roles in immune response, diagnostics, and therapeutics.