6.7

• Antigen


  Cell-surface molecule that can stimulate an immune response, usually (glyco)protein, sometimes (glyco)lipid or polysaccharide
• Immune system


  Recognises antigens as "self" or "non-self", enabling identification of cells from other organisms, pathogens, toxins & abnormal body cells
• Inflammation


  1. Damaged vessels release histamines, causing vasodilation

  2. Blood flow & permeability of blood vessels increase

  3. White blood cells & plasma into the infected tissue
• White blood cells involved in phagocytosis

  • Neutrophils
  • Macrophages (can become antigen-presenting cells)
• Phagocytosis


  1. Phagocyte moves towards pathogen via chemotaxis

  2. Phagocyte engulfs pathogen via endocytosis to form a phagosome

  3. Phagosome fuses with lysosome (phagolysosome)

  4. Lysozymes digest pathogen

  5. Phagocyte absorbs the products from pathogen hydrolysis
• Antigen-presenting cells (APCs)


  Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis), enhancing recognition by TH cells
• Differences between specific and nonspecific immune responses

  • Nonspecific (inflammation, phagocytosis) = same for all pathogens
  • Specific (B & T lymphocytes) = complementary pathogen
  • Nonspecific = immediate
  • Specific = time lag
• Types of specific immune response

  • Cell-mediated
  • Humoral
• Cell-mediated response


  1. Complementary TH lymphocytes bind to foreign antigen on APC

  2. Stimulates clonal expansion of complementary TH cells (become memory cells or trigger humoral response)

  3. Stimulates clonal expansion of cytotoxic T cells (TC) that secrete enzyme perforin to destroy infected cells
• Humoral response


  1. Complementary TH lymphocytes bind to foreign antigen on antigen-presenting T cells

  2. Release cytokines that stimulate clonal expansion of complementary B lymphocytes

  3. B cells differentiate into plasma cells

  4. Plasma cells secrete antibodies with complementary variable region to antigen
• Antibody


  Proteins secreted by plasma cells, with 2 'light chains' held by disulfide bridges and 2 longer 'heavy chains', binding sites on variable region of light chains have specific tertiary structure complementary to an antigen, the rest of the molecule is known as the constant region
• How antibodies lead to the destruction of a pathogen

  • Formation of antigen-antibody complex results in agglutination
  • Activation of complement
  • Opsonisation (marks microbes for phagocytes)
  • Precipitation/neutralisation (makes toxins insoluble)
• Memory cells


  Specialised TH/ B cells produced from primary immune response, remain in low levels in the blood, can divide very rapidly by mitosis if organism encounters the same pathogen again
• Passive immunity


  • Natural: antibodies in breast milk/ across placenta

  Artificial: anti-venom, needle stick injections
• Active immunity


  • Natural: humoral response to infection

  Artificial: vaccination
• Vaccination


  1. Vaccine contains dead/ inactive form of a pathogen or antigen

  2. Triggers primary immune response

  3. Memory cells are produced and remain in the bloodstream, so secondary response is rapid & produces higher concentration of antibodies

  4. Pathogen is destroyed before it causes symptoms
• Herd immunity


  Vaccinating 80-90% of population reduces available carriers of the pathogen to control disease transmission, protects individuals who have not been vaccinated e.g. those with a weak immune system