Human Defence system

• The human body is designed to protect itself from foreign cells or invaders


  Has two lines of defence - general (non-specific) and specific (immune system)
• Immunity


  The ability to resist disease
• Pathogens


  Disease causing agents
• General defence system


  • Skin, blood clotting, lysozyme, mucous membranes, cilia, stomach acid
  • White blood cells, phagocytosis, defensive proteins (interferons and complement), inflammation
• Skin


  • Acts as a structural barrier, secretes chemicals that harm or kill bacteria

  
• Respiratory and digestive tracts, other organs

  • Secrete mucus that removes foreign particles
• Phagocytic white blood cells


  Monocytes and macrophages, second line of defence in general defence system, seek out and destroy cells displaying abnormal molecules

  
• Phagocytosis


  Micro-organisms or abnormal cells are engulfed and destroyed
• Pus


  Collection of dead phagocytic white blood cells and dead bacteria

  
• Complement


  Set of about 20 proteins in blood plasma, activated by infection, involved in a chain reaction that bursts pathogens
• Interferons


  Produced by body cells infected with virus, stimulate nearby cells to prevent viral multiplication, limit spread of virus infection
• Inflammation


  Triggered by release of histamine from damaged cells, causes blood vessels to dilate, brings more blood to site, site becomes red, swollen and tender, high concentration of white cells and tissue fluid with anti-bacterial effect
• Specific defence system (immune system)

  Can attack specific organisms, more efficient than general defence system, involves antigen-antibody response, uses specialised white blood cells - lymphocytes and monocytes
• Organs specific to the immune system

  • Spleen
  • Thymus
  • Lymph nodes
• Antigen


  Foreign molecule that stimulates production of antibodies, found in bacterial cell walls, viral coats, foreign cells, or produced in cancerous cells
• Antibody


  Protein produced by white blood cells (lymphocytes) in response to an antigen
• Lymphocytes


  Types of white blood cells formed in bone marrow, found in lymphatic tissue, react to antigens
• Monocytes


  Develop into macrophages, destroy pathogens and display antigens on their membranes to stimulate antibody production
• Lymphocytes


  • Stored in lymphatic system, have a large nucleus (25% of white cells), produce antibodies
• Antibodies


  Form part of a family of proteins called immunoglobulins, each type is highly specific to a single pathogen, inactivate antigens by attaching to them, causing pathogens to clump together, or activating complement protein
• Induced immunity


  Ability to resist disease caused by specific pathogens through production of antibodies, can be active (person's own antibodies) or passive (antibodies from another organism)
• Natural active immunity


  Occurs when pathogen enters body normally and body produces its own antibodies
• Artificial active immunity


  Occurs when pathogen is medically introduced into body (vaccine), body then produces its own antibodies
• Natural passive immunity


  Antibodies enter body in normal manner, e.g. from mother to baby
• Artificial passive immunity


  Antibodies enter body by injection, e.g. anti-tetanus
• Memory cells


  Antibody producing lymphocytes that remain in body after infection is overcome, can rapidly produce large amounts of specific antibody on second exposure
• Vaccine


  Specially prepared material that provides artificially induced active immunity against a pathogen, usually contains dead or weakened pathogen, close relative, component parts, genetically engineered antigens, or DNA
• Vaccination


  • Provides quicker and greater response to future infection without illness, produces much less severe symptoms than the disease
• Before smallpox was eradicated, it was a serious infectious disease caused by the variola virus, contagious, most people recovered but about 3 in 10 died, many survivors had permanent scars
• Edward Jenner


  Noticed that milkmaids who contracted cowpox were protected from smallpox, developed technique of vaccination
• Vaccination


  Jenner injected 8 year old boy with cowpox, then with smallpox-infected tissue, boy did not contract smallpox, technique was immediately accepted and death rate from smallpox collapsed by end of 19th century
• Lymphocytes


  Continually inspect monocytes and body cells for evidence of "foreign" antigen, when detected they divide into huge numbers
• Types of lymphocytes


  • B-lymphocytes (B-cells)
  • T-lymphocytes (T-cells)
• 1. lymphocytes


  Produced and mature in bone marrow, migrate to lymph nodes, each carries receptors for one specific antigen, when antigen is encountered they divide into plasma cells (produce antibodies) and memory B cells
• Plasma cells


  Produce huge numbers of antibody molecules that circulate and mark foreign cells for destruction by monocytes
• Memory B cells


  Circulate in body for years and intercept antigens much quicker on second attack
• 1. lymphocytes


  Produced in bone marrow, mature in thymus gland, migrate to lymphoid tissue, work against viral infected and cancer cells, form 4 types of clones: helper T-cells, killer T-cells, suppressor T-cells, memory T-cells
• Helper T-cells


  Recognize antigens presented by macrophages and produce interferon, stimulate production of other T-cells and B-cells, greatly increase antibody production
• Killer T-cells


  Destroy virus infected cells, tumour cells and transplant tissue by punching holes in their membranes with the protein perforin
• Suppressor T-cells


  Switch off activity of T and B-cells once pathogen has been destroyed, turn "off" the immune response