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.