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The immune system- Immune response step by step

Foreign Antigens trigger and Immune response

  •   Antigens are molecules( proteins or polysaccharides)
  • found on the surface of cells
  • when pathogen enters body the antigens identifies the cell as forgeign 
  • This activates cells in the immune system
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First stage

Four stages of immune response :

1) Phagocytes (eg. macrophage) Engulf Pathogens

  • they are a type of white blood cell that carries out phagocytosis ( engulfing a pathogen)
  • they are found in tissues and blood
  • first cells to respond to pathogen inside the body
  • First the phagocyte recognises the antigens on the pathogen as non self ( forgeign), 
  • then the cytoplasm of the pathogen moves round the pathogenengulfing it. 
  • The pathogen is now in the phagocytic vacuole in the cytoplasm.
  •  A lysosome fuses with the phagocytic vacuole
  • The lysosomal enzymes break down the pathogen.
  •  The Phagocyte will then present the pathogens antigens on its surface to activate otherimmune system cells.
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Second and Third stage

 2) Phagocytes Activate T-cells

  • Tcells are another type of white blood cell
  • it has protiens that bind to the antigens presented to it by the phagocyte
  • This activates the Tcell - ( some release substances to activate Bcells, Some attach antigens on a pathogen and kill the cell.)

3) Tcells activate B cells, which devide in to plasma cells

  • Bcells are another white blood cell, that are covered in antibodies( that bind to antigens)
  • then form antigen-antibody complex when they bind
  • each B cell has a different shaped antibody
  • When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it.
  • This, as well as substances from Tcells activates the Bcell
  • The activated Bcell divides in to plasma cells.
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4th Stage and Antibody info

4) Plasma cells make more antibodies to a specific antigen 

 They secrete loads of the antibody specific to the anitgen

Functions include:

  • Coating the pathogen to make it easier for a phagocyte to engulf it
  • coating the pathogen to prevent it from entering host cells
  • binding to and nuetralsing ( inactivating) toxins produced by the antigen.


  • protiens
  • made of chains of amino acid monomers
  • linked by peptide bonds
  • the variable regions are what makes them specific ( due to amino acid sequence)
  • complementry to a specfic antigen
  • constant regions are all the same in all antibodies.
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