Microbial Antigens

Antigens (Ag) are any substance that can stimulate the body’s immune system, leading to the production of specific antibodies (Ab). Microbial antigens are molecules originating from microorganisms that can trigger an immune response in a host organism.

I. Classification of Microbial Antigens:

1. Based on Location:

• Soluble Antigens: These antigens are soluble in water and are released from bacteria into their surroundings.
  • Exotoxins: These are potent antigenic proteins released by living bacteria.
    • Note: Exotoxins can be converted into toxoids by treatment with formalin at 40°C for a specific duration. Toxoids retain their antigenicity but lose their toxicity.
    • Examples: Diphtheria and tetanus vaccines utilize toxoids.
  • Extracellular Enzymes: These are enzymes secreted by bacteria that can damage host cells.
    • Example: Hemolysin is an extracellular enzyme produced by Streptococcus bacteria, which can cause hemolysis (red blood cell lysis).
    • The ASLO (Anti-Streptolysin O) serological test is often used to diagnose Streptococcus A infections, based on the production of antibodies against hemolysin.
• Cellular Antigens: These are antigens that constitute the structural components of bacteria.
  • Cell Wall Antigens: Found on the bacterial cell wall.
  • Capsular Antigens: Located in the capsule surrounding the bacterial cell wall.
  • Fimbrial Antigens: Found on bacterial fimbriae (pili).
  • Surface Antigens: Located on the surface of the bacterial cell.

2. Based on Origin:

• Viral Antigens: Found on the structure of viruses.
  • Soluble Viral Antigens: These are components obtained from virus-infected cell cultures after removing the virus and cell components.
    • Examples: Viral enzymes, excess structural components.
  • Structural Viral Antigens: These are the main structural components of the virus.
    • Nucleoprotein Antigens: Formed from nucleic acid and protein.
      • Example: Ribonucleoprotein is a type-specific antigen for influenza viruses.
    • Capsid Antigens: These are the most important viral antigens.
      • Note: The capsid contains most of the viral protein, which may be a distinct antigen or a complex nucleoprotein antigen.
      • Role: Classifying viruses that lack an envelope.
    • Envelope Antigens:
      • Nature: Lipoproteins.
      • Note: Envelopes often contain type-specific antigens like hemagglutinin (H) and neuraminidase (N) (e.g., influenza viruses).

II. Detailed Description of Antigen Types:

1. Cell Wall Antigens:

• Gram-positive:
  • Main Structure: Peptidoglycan (weakly immunogenic).
  • Strong Antigenic Components: Polysaccharides or polypeptides.
  • Roles:
    • Species-specific: Polysaccharides or polypeptides.
    • Specific Agglutination: Protein A of Staphylococcus, protein M of Streptococcus A.
    • Classification: Gram-positive bacteria are categorized into groups, each with different types based on protein A of Staphylococcus and protein M of Streptococcus A.
• Gram-negative:
  • Main Antigenic Components: Lipopolysaccharide (LPS).
  • Roles:
    • Type-specific: LPS determines the specificity of the antigen.
    • Endotoxin: The lipid portion of LPS has endotoxic properties.
  • Note: Cell wall antigens of Gram-negative bacteria are often referred to as somatic antigens.

2. Capsular Antigens:

• Nature: Type-specific polysaccharides (Streptococcus pneumoniae, E. coli) or polypeptides (Yersinia pestis, Bacillus anthracis).

3. Fimbrial Antigens:

• Nature: Protein with specificity.
• Note: Fimbrial antigens are often referred to as H antigens.

4. Surface Antigens:

• Example: Salmonella typhi.
• Nature: Thin polysaccharide layer surrounding the bacterial cell wall.

III. Note:

• Microbial antigens play a crucial role in stimulating the body’s immune system, producing antibodies to fight against bacterial invasion.
• Microbial antigens are used in vaccine production, enabling the body to develop protective immunity against diseases.
• Understanding microbial antigens is essential for researching, diagnosing, and treating bacterial infections.

IV. References:

• Biology Textbook for Class 10 – National Education Publishing House, Vietnam.
• Medical Microbiology – Medical Publishing House, Vietnam.
• Microbiology: An Introduction – Ninth Edition – Pearson Publishing.