CELL RECOGNITION AND IMMUNE SYSTEM

• non-specific defence mechanism

  • response is immediate
  • the same for all pathogens
• specific defence mechanism

  • response is slower
  • specific to each pathogen
• types of non-specific defence mechanisms

  • physical barrier
  • phagocytosis
• types of specific defence mechanisms

  • cell mediated response (T lymphocytes)
  • humoral response (B lymphocytes)
• a non- specific mechanism is a

  barrier to disease
• epidermis of the skin

  • layers of dead cells prevent infection
• ciliated epithelia

  • sweeps invaders away so they can be removed
• mucus membranes

  • protective mucus layer secreted by goblet cells
  • invaders get trapped in mucus
• hydrochloric acid in stomach

  • low pH so enzymes of pathogens are denatured
• the problems with the barriers

  some pathogens can penetrate the barriers
• pathogens penetrating the barriers

  1. malaria is caused by plasmodium passing through the skin when a mosquito bites
  2. buponic plague enters the skin by flea bites
  3. influenza virus passes through the lining of the trachea and lungs
• pathogen

  an organism which causes disease
• toxin

  molecules that are poisons released by the pathogen
• antigen

  • molecule that can generate an immune response
  • usually a protein on the cell surface membrane of a pathogen
  • unique to that pathogen
  • bacteria, virus, fungi, allergen
• phagocytosis

  • process by which pathogens are taken up and destroyed by white blood cells (phagocytes)
  • WBC are continually produced from stem cells in bone marrow and foetal liver
  • stored in bone marrow and released into blood to engulf + digest foreign bodies
• two types of phagocytes

  1. neutrophils
  2. macrophages (monocytes)
• phagocytosis

  1. phagocyte attracted to pathogen's antigens
  2. phagocyte engulfs pathogen
  3. pathogen in vesicle
  4. lysosome fuses with vesicle and releases lysins
  5. pathogen is hydrolysed
• humoral response

  • specific defence mechanism used to protect the body from disease
  • production of specific antibodies to destroy pathogens
• B cells

  • have antibodies on their membrane that bind to the complementary antigens on pathogens
  • they engulf the antigens and display them on their cell-surface to become antigen-presenting cells
  • once activated the B cells divide into plasma and memory cells
• plasma cells

  • types of B cells that can produce and secrete antibodies against a specific antigen
  • have a short lifespan
• memory cells

  • provide long-term immunity against specific pathogens
  • longer lifespan
  • rapidly divide into plasma cells if the body is re-infected by the same pathogen
• helper T cells

  • bind to the antigen-presenting cell to activate the mitosis of B cells
• humoral response

  1. a B cell with a complementary antibody binds to the antigen on the pathogen
  2. the B cell engulfs the pathogen and presents its antigens on the membrane to become an antigen-presenting cell
  3. CLONAL SELECTION - activated T helper cell binds to the B cell causing mitosis
  4. CLONAL EXPANSION - divides into plasma and memory cell clones
  5. cloned plasma cells produce and secrete specific antibodies
  6. memory cells circulate the blood, ready to divide into plasma cells if the body is re-infected by the same pathogen
• clonal selection

  • the B cell with the correct antibody is selected for cloning
• cloning expansion

  • division of specific B cell to produce identical clones
• primary immune response

  1. production of antibodies is slow after the first exposure to the pathogen
  2. concentration of antibodies increases slowly
  3. very few B cells that are specific to the pathogen's antigens
  4. takes time for the B cells to divide into plasma cells to produce the correct antibody
  5. during this process, some B cells divide into memory cells
• secondary immune response

  1. the production of antibodies is much quicker after exposure
  2. the concentration of antibodies increases quickly
  3. this is due to memory cells recognising the pathogen's antigens and quickly divide into plasma cells
  4. plasma cells secrete a larger number of antibodies to quickly destroy the pathogen
• T cells

  mature in the thymus gland
• T cells are involved in the

  cellular response, responding to antigens presented on body cells
• helper T cells

  • receptors on their cell-surface
  • bind to complementary antigens on antigen-presenting cells
  • after binding, they form memory cells, stimulate B cells, and activate cytotoxic T cells
• cytotoxic cells

  • kill abnormal cells by producing a protein called perforin
  • makes holes in the cell-surface membrane
  • becomes freely permeable, and dies
• memory T cells

  • long term immunity against specific pathogens
  • rapid response if the body is re-infected as can differentiate into T killer cells
• cellular response

  1. phagocytes engulf the pathogens and display their antigens on the cell surface (antigen-presenting cell)
  2. helper T cells with complementary receptors bind to these antigens
  3. the helper T cell is activated to divide by mitosis to form genetically identical cells
  4. T helper cells produce interleukins
  5. interleukins trigger phagocytes, trigger killer T cells, trigger B lymphocytes to divide
• humoral response

  production of antibodies to help destroy pathogens
• antibody structure

  • Y shaped glycoproteins
  • 4 polypeptide chains
  • 2 heavy chain
  • 2 light chains

  
• constant region

  same for all the antibodies

  bind to receptors on cells such as B cells
• variable region

  different for each antibody

  shape is complementary to a specific antigen

  part of the antibody that binds to antigens
• antibody function

  bind to a specific antigen on a pathogen's surface
• binding sites

  each antibody has a unique binding site (on variable region) that fits onto a specific antigen

  when an antibody fits to an antigen - they from an antigen-antibody complex

  
• agglutination of pathogens

  • antibodies act as agglutinins, causing pathogens to clump together
  • making it easier for phagocytosis to locate the pathogens
  • and so they can engulf a number of pathogens at once