Pathogens vs Hosts: An Evolutionary Race (Edexcel International A Level Biology)

• Vertebrates have evolved over millions of years to have immune systems that are capable of dealing with a wide range of different pathogens
• Pathogens, however, have also evolved and have developed different ways of evading their host's immune system
• This battle between host and pathogen is known as an evolutionary race; each organism develops new ways in which to have an advantage over the other
  • This evolutionary race is sometimes referred to as an evolutionary arms race
• Evasion mechanisms developed by pathogens serve as support for this theory

HIV evasion mechanisms

• The virus kills helper T cells after it infects them which reduces the number of cells that could detect the presence of the virus and activate the production of antibodies
• HIV shows antigenic variability due to the high mutation rate in the genes coding for antigen proteins
  • This forms new strains of the virus which each require a new primary immune response
    • Memory cells for one strain will not recognise the antigens of another strain
• The virus prevents infected cells from presenting their antigens on the cell surface membrane, making it very difficult for the relevant white blood cells to recognise and destroy the infected cells

Mycobacterium tuberculosis evasion mechanisms

• Once engulfed by phagocytes in the lungs the bacteria produce substances that will prevent a lysosome from fusing with the phagocytic vacuole
  • This prevents the bacteria from being broken down by digestive enzymes, leaving them to multiply within the phagocyte
• As with HIV the bacteria can disrupt antigen presentation in infected phagocytes, making it difficult for the immune system to recognise and destroy these cells