Cell recognition

• How do we prevent pathogens entry

  By using physical and chemical barriers such as skin ,mucous membranes , tears containing lysozome(enzyme that kills bacteria) , saliva and stomach acid
• what are antigens 


  protein molecules

  allow cell to cell recognition

  part of phospholipid bilayer can be glycoprotein or glycolipid

  allows body to recognise pathogenic cells , abnormal cells . toxins (chemical rather than cellular )

  can be found on cell surface membrane , bacteria cell wall or virus surface
• importance of antigens in phagocytosis

  know as phagocyte cell , has surface protein that acts as receptors and bind to proteins (antigens on pathogens surface ) enables pathogen to be digested . antigens now presented on phagocyte surface to trigger specific immune response
• what are self antigens 


  produced by organisms own cells and do not cause a immune response
• what are non self antigens

  antigens not produced by organisms and trigger a immune response
• Problems of antigen variability

  Antigens on surfaces can change due to mutation

  surface receptor on lymphocytes and memory cells only complementary to specific antigen shape

  changed antigen no longer binds and does not trigger secondary immune response
• what are phagocytes 


  white blood cells produced in bone marrow

  distributed around body

  remove dead cells and invasive microorganisms

  carry out non specific immune response

  two types neutrophils and macrophages
• how do neutrophils work

  Squeeze through capillary walls , released in large numbers during infection

  chemicals released by pathogen attract neutrophils (chemotaxis)

  antigens on pathogen trigger neutrophils to attack as neutrophils have complimentary receptors and attach to pathogen

  once attached cell surface membrane extends and engulfs pathogen (endocytosis)
• what is formed after a phagocyte engulfs a pathogen

  phagosome
• Exam Q how do phagocytes destroy pathogens

  pathogen releases chemical that attracts phagocyte

  phagocyte has complimentary receptors and binds to pathogen antigens on cell surface membrane

  phagocyte then engulfs pathogen and forms phagosome

  lysosomes in phagocyte bind to pathogen to form phagolysome .

  lysosomes release hydrolytic enzyme that destroys pathogen

  phagocyte absorbs dead cell and antigens placed on cell surface membrane

  forms antigen presenting phagocyte
• how do macrophages work

  larger than neutrophils and long lived

  do not remain in blood and move into organs

  after production travel in blood as monocytes which develop into macrophages once out of blood

  carry out phagocytosis similarly but display pathogens antigens on surface which can then be recognised by lymphocytes
• lymphocytes fact sheet 


  another type of white blood cell

  smaller than phagocyte

  produced in bone marrow

  have large nucleus

  two types helper T cels and B lymphocytes
• how are T cells activated

  activated when they encounter specific antigen presented on host cells like a macrophage

  Activated T lymphocytes divide by mitosis to increase number

  differentiate into Helper T cells and cytotoxic cells (Killer T cells )
• Role of Helper T cells

  assist other white blood cells in immune response

  release cytokines which stimulate maturation of B lymphocytes into antibody secreting plasma cells

  produce memory. cells

  activation of cytotoxic T cells which destroy virus infected cells and tumour cells

  increase rate of phagocytosis
• role of T killer cells 


  patrol body in search of antigen presenting body cells

  attach to foreign antigens on cell surface membranes and secrete toxic substances that kill infected cells and pathogen

  perforins secreted by T killer cells puncture hole in cell surface membranes of infected cell allowing toxins to enter
• B lymphocytes fact sheet

  produced in bone marrow

  as they mature genes coding for antibodies changed to code for different antibody

  once mature each type of B-lymphocyte can only produce one antibody type and remain in cell surface membrane

  part of each antibody molecule forms glycoprotein receptor that is complimentary to one antigen

  if antigen present B lymphocyte cells with correct receptors recognise it and bind ( clonal selection) releasing antibody

  two types plasma cells and memory cells
• primary immune response 


  when antigen enters body b lymphocytes with complimentary receptors stimulated to divide by mitosis

  increases numbers

  some lymphocytes become plasma cells and secrete antibody molecules into blood and body

  plasma cells short lived but antibodies secreted stay in blood for longer

  other lymphocytes stay as memory cells which last longer and speed up process of clonal expansion and immune response quicker
• structure of antibodies 


  globular proteins called immunoglobulins

  have a quaternary structure (Y shape) which two heavy polypeptide chains on end by disulphide bonds to two light short polypeptide chains

  polypeptide has a constant and variable regions

  amino acid sequence in variable regions (tips of Y)are different for each antibody and is where antigen antibody complex is formed
• structure of antibodies pt 2

  variable regions contain antigen binding site and vary greatly giving specific shape to antigens

  hinge region (disulphide bridge) gives flexibility to antibody allowing antigen binding site to be at different angle
• role of antibodies 


  neutralise pathogen or acts as a marker to attract phagocytes to engulf and destroy pathogen

  cause agglutination which clumps pathogens making it easier for phagocytosis . this is because they have more than one binding site so can bind to more than one pathogen at a time
• process of secondary immune response

  if same antigen encountered again memory cells recognise antigen and divide quickly and differentiate into plasma cells .

  response very quick and pathogen removed before numbers increase to add harm