The Immune Response


Swelling/ Inflammation

  • Infected cells release chemicals (e.g. histamine) attracting neutrophils
  • Histamine also causes capillaries to become more leaky, meaning more fluid leaves the area of infection (making it red & swollen)
  • This means more tissue fluid passes into the lymphatic system, leading pathogens towards macrophages waiting in the lymph nodes
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  • When pathogen enters body it is recognised as foreign by antigens on its outer membrane
  • Antibodies attach to foreign antigens and membrane-bound receptors on phagocyte bind to antibodies
  • Phagocyte then engulfs pathogen, folding the membrane inwards, trapping the pathogen in a phagosome
  • Lysosomes fuse with the phagosome (forming a phagolysosome) and release lysins into it which digest the bacterium
  • Harmless end products of digestion are then absorbed by the cytoplasm
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Activation & expansion of T lymphocytes

Clonal selection

  • Selection of correct T lymphocytes for immune response
  • Antigen presentation first takes place by phagocytes and cells attacked by pathogen (don't fully digest pathogen & incorporate its antigens into cell surface to attract lymphocytes)
  • T lymphocyte receptors bind to these antigens, activating the T lymphocyte

Clonal expansion

  • T lymphocyte divides by mitosis to produce clones (and increase numbers to become effective) which then differentiate into different types of lymphocytes with different functions; 
  • T helper cells - release cytokines which stimulate differentiation of B lymphocytes
  • T memory cells - stay in the body for years and act as immunological memory, v. active in secondary response 
  • T killer cells - search body for pathogens, attach to foreign antigens, and produce toxic substances
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Activation & expansion of B lymphocytes

Clonal selection

  • Correct B lymphocytes are first selected for (like T lymphocytes) the immune response by clonal selection

Clonal expansion

  • T helper cells release cytokines which stimulate the differentiation of the B lymphocytes by cell signalling (T cells are signalling to B cells that there is a pathogen in the body)
  • Activated B cell divides by mitosis into;
  • B plasma cells - manufacture and release antibodies very quickly into the blood
  • B memory cells - remain in the body for years and act as immunological memory, respond v. quickly to secondary response
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Action of plasma cells

  • Manufacture and release antibodies into the blood, specific to the antigen
  • Antibodies bind to the antigens on the surface of the pathogen , forming antigen-antibody complexes


  • Large antibody binds many pathogens together
  • The group of pathogens is too large to enter the host cell


  • Antibodies bind to pathogen's antigens (binding sites) thus preventing the pathogen from binding to and entering a host cell
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