Cells & the immune system

• What is an antigen?

  Molecules (usually proteins) that can generate an immune response when detected by body. Usually on cell surface. Allow immune system to identify pathogens, abnormal body cells, toxins & other foreign cells.
• What is a phagocyte?

  Type of white blood cell found in blood & tissues. Carries out phagocytosis (engulfment of pathogens). 1st cells to respond to an immune system trigger.
• Explain phagocytosis:


  Phagocyte recognises foreign antigens on pathogen. Cytoplasm of phagocyte moves round pathogen, engulfing it. Pathogen now contained in phagocytic vacuole in cytoplasm of phagocyte. Lysosome fuses with phagocytic vacuole. Lysozymes break down pathogen. Phagocyte presents pathogen's antigens on its surface to activate other immune system cells. Acts as antigen-presenting cell.
• What are T-cells?


  Type of white blood cell. Has receptor proteins on its surface that bind to complementary antigens presented by phagocytes. This activates T-cell.
• What are helper T-cells?

  Release chemical signals that activate & stimulate phagocytes & cytotoxic T-cells which kill abnormal & foreign cells. Activate B-cells which secrete antibodies.
• What are B-cells?


  White blood cell covered with antibodies (proteins that bind to antigens to form complex). Each B-cell has different shaped antibody on its membrane, so different ones bind to different shaped antigens. Receptor proteins on surface that bind to signalling molecules released by the T-helper cells.
• How do B-cells work?

  When its antibody binds to complementary shaped antigen, B-cell activated-done together with substances released from T-helper cells. Activated B-cell divides into plasma cells by mitosis (clonal selection). Cells produced are genetically identical so their antibodies are specific to pathogen. Plasma cells secrete antibodies.
• Structure of an antibody:

  2 binding sites-can bind to 2 pathogens at same time (pathogens clumped together (agglutination)). Phagocytes bind to antibodies & phagocytose many pathogens at once. Antibodies made of amino acid chains. Specificity of antibody depends on its variable regions which form antigen binding sites. Each antibody has variable region with unique tertiary structure complementary to a specific antigen. All antibodies have same constant regions. Contains heavy & light chains, hinge region & disulfide bridge.
• How many polypeptide chains do antibodies have?

  4 chains held by disulphide bridges. 2 heavy chains & 2 light chains.
• What is the hinge region for?

  Allows antibody to flex in order to bind to more than 1 antigen.
• What is the cellular response?

  T-cells & other immune system cells they interact with e.g, phagocytes. T-cells mature in thymus gland.
• What is the humoral response?

  B-cells, clonal selection & production of monoclonal antibodies. B-cells mature in bone marrow.
• What is the primary immune response?

  When antigen enters body for first time, it activates immune system. Slow response as there aren't many B-cells that can make antibody needed to bind to it. Eventually body produces enough of right antibody to overcome the infection. Has symptoms of disease. After being exposed to an antigen, both T & B-cells produce memory cells. Memory cells remain in body for long time. Now immune.
• What do memory T-cells remember?


  Specific antigen & recognises it 2nd time round.
• What do memory B-cells remember?

  Record specific antibodies needed to bind the antigen.
• What is the secondary immune response?

  If same pathogen enters body again, immune system produces quicker, stronger immune response. Clonal selection happens faster. Memory B-cells activated & divide into plasma cells that produce right antibody to antigen. Memory T-cells activated & divide into correct type of T-cells to kill cell carrying antigen. Often gets rid of pathogen before symptoms begin to show.
• Uses of monoclonal antibodies:


  Targeted medication & medical diagnosis.
• What is active immunity?

  Immunity you get when immune system makes its own antibodies after being stimulated by an antigen.
• What is natural active immunity?


  Become immune after catching a disease.
• What is artificial active immunity?

  Become immune after being given vaccination containing harmless dose of antigen.
• What is passive immunity?


  Immunity you get from being given antibodies made by a different organism.
• What is natural passive immunity?

  Baby becomes immune due to the antibodies it receives from its mother, through the placenta & in breast milk.
• What is artificial passive immunity?


  Become immune after being injected with antibodies from someone else.
• Differences between active & passive immunity:

  Active requires exposure to antigen-passive doesn't. Active takes while for protection to develop-protection is immediate in passive. Memory cells produced in active-aren't produced in passive. Long-term protection in active as antibody produced in response to complementary antigen. Short-term protection in passive as antibodies given are broken down.
• What is herd immunity?

  Unvaccinated people are protected because the occurrence of the disease is reduced by the number of people who are vaccinated.
• What are monoclonal antibodies?

  Antibodies produced from a single group of genetically identical B-cells (plasma cells).
• What do vaccines contain?

  Antigens that cause body to produce memory cells against particular pathogen without pathogen causing disease. Become immune without getting symptoms. Antigens may be free or attached to dead or attenuated (weakened) pathogen.
• Disadvantage of oral vaccines?

  Could be broken down by enzymes in gut or molecules of vaccine may be too large to be absorbed into blood.
• Ethical issues of vaccines:


  Tested on animals, animal based substances may be used to produce vaccine, volunteers may put themselves at risk of contracting disease as they think they're fully protected, side effects, if there was epidemic of new disease there'd be rush to receive vaccine & difficult decisions as to who first receives it.
• What is antigenic variation?


  Pathogens can change their surface antigens. Hard to develop vaccines against them.
• Features of an effective vaccination programme:

  Few side effects, easy to administer, able to produce, store & transport, needs to produce herd immunity (possible to vaccinate most of population) & economically available in sufficient quantities to immunise most of vulnerable population.
• What is HIV?


  Virus that affects human immune system. Leads to AIDS (immune system deteriorates & eventually fails). Makes someone more vulnerable to other infections e.g, pneumonia.
• How does HIV work?

  Infects & eventually kills T-helper cells, which act as host cells. Develop AIDS when T-helper cells in body reach critically low level. Immune system unable to mount effective response. During initial infection period, HIV replicates rapidly & infected person experiences flu-like symptoms. After this, HIV replication drops to lower level (latency period). Can last for years & won't have symptoms.
• What are the symptoms of AIDS?

  Develop diseases that wouldn't cause serious problems in people with healthy immune system. Initial symptoms: minor infections of mucous membranes & recurring respiratory infections. Immune system cell number decreases & patient susceptible to more serious infections.
• What can affect survival time with AIDS?

  Existing infections, strain of HIV, age & access to healthcare.
• HIV structure:


  Virus particle has spherical structure. Core contains genetic material (RNA), some proteins & enzyme reverse transcriptase. Outer coating of protein (capsid) & extra outer layer (envelope) made of membrane from previous host cell. Attachment proteins sticking out of envelope.
• Describe HIV replication:


  Attachment protein attaches to receptor molecule on cell membrane of host helper T-cell. Capsid released into cell where it uncoats & releases genetic material (RNA) into cell's cytoplasm. Reverse transcriptase makes complementary strand of DNA from viral RNA template. Double-stranded DNA made & inserted into human DNA. Host cell enzymes make viral proteins from viral DNA in human DNA. Proteins make new viruses which bud from cell & infect other cells.
• Why can't antibiotics be used against viruses?

  Human viruses use human enzymes & ribosomes in host's cells to replicate. Therefore antibiotics can't inhibit them as they don't target human processes.
• How do antiviral drugs work against viruses?

  Drugs target the few virus-specific enzymes. HIV uses reverse transcriptase enzyme to replicate. Human cells don't use this enzyme, so drugs can inhibit it without affecting host cell. Slows down progression of HIV & AIDS.
• How can HIV be spread?

  Unprotected sex, infected bodily fluids & HIV-positive mother to child.