Antigenic variation

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  Mechanisms of VSG switching

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  Mutation outside b12 epitope affects binding of b12 to monomeric gp120

Antigenic Variation

Antigenic variation is a mechanism by which an infectious organism alters its surface proteins in order to evade a host immune response. This process is a significant factor in the persistence of infections and the ability of pathogens to cause recurrent infections. Antigenic variation is observed in various pathogens, including viruses, bacteria, and protozoa.

Mechanisms of Antigenic Variation

Antigenic variation can occur through several mechanisms, including:

• Gene conversion: This involves the replacement of a gene segment with a homologous sequence from a different gene, leading to the expression of a new antigenic variant. This mechanism is commonly seen in Trypanosoma brucei, the causative agent of African sleeping sickness.

• Site-specific recombination: This process involves the rearrangement of genetic material at specific sites within the genome, resulting in the expression of different antigenic variants. An example of this is seen in Borrelia burgdorferi, the bacterium responsible for Lyme disease.

• Point mutations: These are small changes in the DNA sequence that can lead to changes in the amino acid sequence of proteins, resulting in antigenic variation. This mechanism is frequently observed in influenza virus and is responsible for the seasonal variation in flu strains.

• Phase variation: This involves the reversible switching on and off of gene expression, leading to changes in the surface antigens. This mechanism is seen in Neisseria gonorrhoeae, the bacterium that causes gonorrhea.

Importance in Pathogenesis

Antigenic variation plays a crucial role in the pathogenesis of infectious diseases. By altering their surface antigens, pathogens can:

• Evade the host's immune system, allowing them to persist in the host for extended periods.
• Cause recurrent infections, as the immune system may not recognize the new antigenic variants.
• Complicate vaccine development, as vaccines may not be effective against all antigenic variants.

Examples of Antigenic Variation

• Trypanosoma brucei: This protozoan parasite undergoes antigenic variation through gene conversion, allowing it to evade the host's immune response and cause chronic infections.

• Plasmodium falciparum: The causative agent of malaria, this parasite exhibits antigenic variation in its PfEMP1 proteins, which are expressed on the surface of infected red blood cells.

• Influenza virus: The virus undergoes antigenic drift and shift, leading to the emergence of new strains that can evade pre-existing immunity in the population.

• Neisseria gonorrhoeae: This bacterium uses phase variation to alter its pili and outer membrane proteins, helping it to evade immune detection.

Related Pages

• Immune system
• Pathogen
• Vaccine
• Infectious disease