2.4 cell recognition and the immune system

• What is a pathogen?

  An infectious agent that causes disease or illness in a host.
• What is an antigen?

  A foreign/non-self protein that stimulates an immune response or the production of antibodies.
• Where are antigens found?

  • pathogens
  • toxins including those produced by pathogens
  • abnormal body cells e.g. cancer cells
  • non-self cells e.g. donor cells (if you have an organ transplant)
• How do you minimise tissue rejection from an organ transplant?

  • match the donor and recipient‘s self receptors, the donor is usually someone close in relation to you as the closer the donor and recipient are, the higher the likelihood that they will have the same/similar self receptors
  • use immuno-suppressive drugs which would prevent and suppress the person’s immune response
• What are the two mechanisms of disease that pathogens utilise?

  • releases toxins (chemicals that interfere with the normal biochemical reactions in cells)
  • kills/damages cells or tissues
• What are the five different kingdoms of pathogens?

  • viruses
  • bacteria
  • fungi
  • protists
  • prions
• What is an antibody?

  It is a protein that is specific to an antigen, and is produced by B plasma cells.
• What is another name for antibodies?

  Immunoglobulins
• What is the main structure of an antibody?

  Y-shaped protein.
• How are antigen-antibody complexes formed?

  • antigens and antibodies are able to bind together
  • the variable region on the antibody has a specific amino acid sequence
  • the tertiary structure of the binding site is complementary to specific antigens
  • this forms an antigen-antibody complex
• What do antigen-antibody complexes lead to?

  • agglutination of cells (stick pathogens together in one place) - easier for phagocytes to locate + destroy them
  • markers that stimulate phagocytes to engulf the pathogen
  • neutralises toxins - stops them, from interfering with cell reactions
• On this diagram what does the S represent?

  Disulphide bonds

  
• What type of protein structure does an antibody have?

  Quaternary structure as it is made of 4 polypeptide chains.
• What part of the antibody differs from antibody to antibody?

  The variable regions which have different sequences of amino acids and so different tertiary structures.
• How do vaccines protect us against pathogens?

  • antigen binds to surface receptor on a specific T cell
  • T cell stimulates/activates a specific B cell to divide by mitosis and produce plasma and memory cells
  • B plasma cells release antibodies
  • some B cells become memory cells
  • memory cells produce a larger amount of plasma cells/antibodies faster when the antigen is encountered again
• What is herd immunity?

  When a large proportion of the population are vaccinated to prevent the spread of a pathogen.
• What is a vaccine?

  A substance that contains antigens that stimulates the production of antibodies/plasma/memory cells.
• Why are vaccines sometimes ineffective?

  • pathogens may have multiple strains (pathogens may mutate - antigenic variability)
  • the pathogens conceal themselves from the immune system (e.g. the antigens on a bacterial cell can be concealed through the presence of a slime capsule - this would mean the immune system would be unable to detect the bacterial cell)
  • may develop disease immediately after having the vaccine (haven’t had enough time to create memory cells and antibodies)
  • individuals may object to vaccines (religion/ethical - vaccines are tested on animals first)
• What does the success of a vaccine depend on?

  • must be economically available to immunise most of the vulnerable population
  • few side effects
  • ability to produce, store and transport (e.g. requires hygienic conditions, refrigerated transport)
  • must be possible to vaccinate the vast majority of the population to produce herd immunity
  • training staff to administer the vaccine properly
• Who cannot always be vaccinated?

  • babies/very young children
  • pregnant women
  • elderly people
  • people with a compromised immune system
• What is active immunity?

  The immune system is stimulated to produce antibodies.
• What is passive immunity?

  When an individual obtains immunity by receiving antibodies, without stimulating an immune response.
• What are two examples of active immunity?

  • infected with pathogen
  • injected with vaccine
• What are three examples of passive immunity?

  • injections (anti-venom)
  • across the placenta
  • ingested via breastmilk
• What are two cons of passive immunity?

  • immunity is short-lived
  • no memory cells are produced (so if the individual were to come into contact with the pathogen again the primary immune response would have to happen again)
• What is the pro of passive immunity?

  Immediate protection.
• What are two pros of active immunity?

  • immunity is long-lasting
  • memory cells are produced (so the individual would have that faster secondary response)
• What is a con of active immunity?

  It takes time to develop immunity.
• Why do you still feel the symptoms of a disease during the primary immune response?

  It takes a while for the correct T-helper cells to be produced and in that time, the pathogen releases toxins and kills/damages cells or tissues.
• What is the primary immune response?

  The first time you are exposed to a new pathogen:

  • phagocytosis
  • complementary T-cells bind and are activated to divide by mitosis
  • clonal selection/mitosis of B-cells (plasma and memory)
• How does the secondary immune response compare to the primary one?

  The response is stronger (more antibodies produced) and faster.
• What are monoclonal antibodies?

  Antibodies with the same tertiary structure (so the same shaped antigen binding site) and cloned from identical B-plasma cells.
• What is direct monoclonal antibody therapy?

  • specific antigens on cancer cells
  • complementary monoclonal antibodies can be given to a patient
  • antigen-antibody complexes formed
  • cancer cell can no longer replicate at an uncontrolled rate
• What are the advantages of using monoclonal antibody therapy?

  It directly targets cancer cells so it doesn’t affect healthy cells and so there are no side effects like hair loss that you would experience with chemotherapy.
• What is the structure of HIV?

  It‘s a retrovirus containing:

  • lipid envelope
  • RNA as genetic material
  • protein capsid
  • attachment proteins
  • reverse transcriptase
• What is the function of reverse transcriptase?

  Catalyses the production of DNA from RNA.
• Describe phagocytosis?

  • the phagocyte is attracted by chemicals released by pathogens/recognises foreign antigen
  • phagocyte engulfs pathogen
  • pathogen enclosed in a vesicle/phagosome
  • the vesicle/phagosome fuses with lysosome
  • lysosome contains digestive enzymes that hydrolyse the pathogen
  • the pathogen’s antigens are presented on the cell membrane of the phagocyte
• What is the role of T-cytotoxic cells?

  • produce perforin protein that creates pores in the cell membrane of pathogens
  • the pathogen is now permeable and the pathogen dies as water leaves and no metabolic reactions can occur
• Where do T cells mature?

  Thymus
• Where do B cells mature?

  Bone marrow