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Pathogens and the immune system

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Specific immune system: cell-mediated response

The cell-mediated response is triggered by cells that have changed in some way rather than by the pathogens themselves. This is the mechanism the immune system uses to remove cells infected with viruses or other pathogens – the humoral response is less good at detecting and dealing with pathogens inside the body cells. The cell-mediated immune system also detects and destroys cells that have mutations that might lead to cancer.

T helper cell activation

T helper cell activation plays an important part in the cell-mediated response. It happens in the same way for both the humoral and the cell-mediated immune response.

The cell-mediated response

  • When a body cell is infected with a bacterium or virus, some of the pathogens are digested and the surface antigens become bound to an MHC in a similar process to that seen in macrophages. As a result the body cell effectively becomes an APC – but it is still infected by the pathogen.
  • T killer cells with complementary receptor proteins bind to the infected APC.
  • If the T killer cells are exposed to cytokines from an active T helper cell, they undergo a rapid series of cell divisions to produce a clone of identical T killer cells.
  • These active T killer cells bind to infected body cells where they release enzymes that digest the membranes of the infected cells. These enzymes produce holes in the membranes so water moves in freely and the cells swell and burst.
  • If any pathogens are released intact when the cell bursts, they are labelled with antibodies produced by the plasma cells, and then destroyed.