• 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