2.4 cell recognition and the immune system

• each type of cell has specific molecules on its surface that identify it
• non-self antigens

  • pathogens
  • cells from organisms of the same species
  • abnormal body cells
  • toxins
• antigen: a molecule (usually a protein) that triggers an immune response
• phagocytosis

  • non-specific --> response is immediate and the same for all pathogens
• phagocytes are a type of white blood cell
• phagocytosis process step 1 to 3

  1. chemical products from non-self antigens attract phagocytes to move toward them
  2. receptors on phagocyte cell surface membrane recognise and attach to chemicals on pathogen surface
  3. engulf the pathogen by endocytosis to form a vesicle (phagosome) from the cell surface membrane
• phagocytosis process step 4 to 6

  4. lysosomes fuse with vesicle forming a phagolysosome

  5. lysozyme (enzymes) in lysosomes destroy ingested bacteria by hydrolysis of their cell walls

  6. soluble products from pathogen are absorbed into the phagocyte cytoplasm, indigestible material is discharged from the cell via exocytosis, and non-self antigens are presented at the cell surface membrane
• specific immune responses

  • slower
  • specific to each pathogen --> responds to antigens
• antigen variability: the shape of the antigens found on a pathogen can vary as a result of mutation

  this can enable a pathogen to evade the immune system
• lymphocytes - a type fo white blood cell

  T - cells = cellular response

  B - cells = humoral response
• cellular response

  1. a body cell displays non-self antigens at cell surface membrane
  2. Th (helper) cell with a complementary shaped receptor binds to a non-self antigen
  3. the Th cell divides by mitosis and releases cytokines
  4. cytokines stimulate Tc (cytotoxic) cells to divide by mitosis and release perforin
  5. cytokines also stimulate further phagocytosis and division of B cells
  6. perforin creates holes in the cell surface membrane of body cells displaying non-self antigens, destroying them
• humoral response

  1. B cells take up non-self antigens and display them at their cell surface membrane
  2. Th cells with complementary receptors bind to the non-self antigens on B cells
  3. Th cells release cytokines
  4. cytokines trigger the selected B cells to clone and differentiate into plasma cells and memory cells
  5. plasma cells release antibodies
  6. antibodies bind to the non-self antigens, forming antigen-antibody complexes
  7. pathogens are clumped together for phagocytosis (agglutination)
  8. memory cells remain ready to clone and release quickly release antibodies if same non-self antigen is encountered
• antibodies

  • immunoglobulins (proteins) specific to an antigen
  • secreted by plasma cells
• antibody structure

  1 - variable region

  2 - antigen binding sites

  3 - constant region

  4 - light chains

  5 - heavy chains

  6 - disulphide bridges

  7 - receptor binding site

  
• antibodies use

  • bind to complementary shaped antigens, forming an antigen-antibody complex
  • this causes agglutination, which enables more efficient phagocytosis
  • base of antibody (receptor binding site) attaches to phagocytes
  • phagocytes engulfs antibodies with the antigens
• vaccines

  • contain a small amount of antigens from one or more pathogen
  • or isolated antigens
• passive immunity

  • produced by the introduction of antibodies into individuals from an outside source
  • no direct contact with pathogen or its antigen
  • no memory cells
  • immunity acquired immediately
• active immunity

  • produced by stimulating the production of antibodies of an individual's own immune system
  • creates memory cells
  • direct contact with pathogen or its antigen
  • immunity takes time to develope
  • long lasting
• herd immunity

  • what: when immunity comes from a sufficiently large proportion of the population being vaccinated
• herd immunity how

  • pathogens are passed from person to person when in close contact
  • if the vast majority of the population is immune, it reduces the likelihood of someone not immune coming into contact with an infected person and contracting the pathogen
• HIV - human immunodeficiency virus
• HIV structure

  1 - RNA - genetic material which codes for virus proteins

  2 - reverse transcriptase - enzyme which acts inside host cell to make DNA copy of RNA genome

  3 - capsid - capsule surrounding virus genome

  4 - viral envelope - phospholipid bilayer

  5 - glycoprotein - surface molecule which aids virus attachment and entry into host cell

  
• HIV replication in T helper cells

  1. HIV attachment proteins bind to receptors on T h cells
  2. capsid enters T h cell and is broken down
  3. viral RNA and reverse transcriptase are released from the capsid
  4. reverse transcriptase catalyses the production of viral DNA from RNA
  5. viral DNA is taken up into the T h cell's genome
  6. T h cell produces viral proteins and replicates viral genetic material
  7. new virus particles are released from T h cell. these then infect more T h cells
• How HIV causes AIDS symptoms

  • HIV specifically attacks T helper cells
  • without a sufficient number of Th cells, the immune system cannot stimulate B cells to produce antibodies or T cytotoxic cells to release perforin to kill infected cells
  • as a result, the body is vulnerable to other infections and cancers
• monoclonal antibodies are antibodies produced by a single clone of B lymphocytes that bind to a single type of antigen molecule
• monoclonal antibody uses

  • medical diagnosis (pregnancy tests)
  • targeting medication to specific cell types by attaching therapeutic drugs to an antibody (cancer treatments)
• monoclonal antibody advantages

  • only small amounts of chemical needed
  • non-target body cells are unaffected as they do not have complementary antigens
• monoclonal antibody ethical issues

  • use of mice in production (deliberately inducing cancer)
  • side effects (death in some MS patients)
  • drug testing concerns
• ELISA tests use antibodies to detect the presence and concentration of a specific protein in a sample solution (e.g. blood)
• ELISA test process

  1. wells of testing plate are coated with antigen of interest
  2. add patient sample --> if antibodies specific to the antigen are present, they will bind to the antigen fixed to edges
  3. well is washed out to remove unbound antibodies
  4. enzyme-conjugated antibodies which are specific to the first antibody are added and bind to first antibody
  5. well washed out to remove unbound enzyme-conjugated antibodies
  6. dye solution added --> enzymes on enzyme-conjugated antibodies will generate a visible colour as particles form E-S complex