Cell recognition and immunity system

• Antigens: foreign proteins on the surface of cells that stimulate an immune response
• Antigens allow the immune system to identify:

  • pathogens
  • abnormal body cells
  • toxins
  • cells from other organisms of the same species
• Phagocytosis

  1. The chemicals released by pathogens attract a phagocyte (chemotaxis)
  2. A phagocyte recognises foreign antigens and binds to them
  3. The phagocyte engulfs the pathogen, forming a phagosome (endocytosis)
  4. A lysosome fuses with the phagosome, releasing lysozymes to digest the pathogen
  5. The phagocyte displays the pathogen's antigens on its surface, becoming an antigen-presenting cell (APC)
• Cellular response

  1. Helper T cells with complementary receptors bind to specific antigens on an antigen-presenting cell
  2. Helper T cells are activated to divide by mitosis to form clones:
  3. The clones stimulate phagocytosis, cytotoxic T cells, or B cells
• Humoral response

  1. A B cell with complementary antibodies bind to antigens on a pathogen
  2. The B cell engulfs the pathogen and presents the antigens on its surface
  3. An activated helper T cell binds to the antigen on the B cell's surface, which activates the B cell (clonal selection)
  4. The activated B cell divides by mitosis to form clones of plasma cells and memory cells (clonal expansion)
• Humoral response

  5. Cloned plasma cells produce monoclonal antibodies specific to the antigen

  6. Antibody binds to antigen. forming an antigen-antibody complex

  7. This allows pathogens to clump together (agglutination), which makes it easier for phagocytes to locate pathogens and engulf many of them at once
• Antibody:

  • a protein specific to an antigen, produced by plasma cells
  • made up of 4 polypeptide chains (2 heavy and 2 light), which are held together by disulfide bridges
• Antibody structure

  A) light chain

  B) heavy chain

  C) disulfide bridges

  D) variable region

  E) constant region

  
• Primary immune response:

  • responding to a newly encountered antigen
  • antibody production is slow: there are very few B cells that are specific to the pathogen's antigen, so it takes time for B cells to divide into plasma cells
  • during this time, the individual experiences symptoms of the disease
  • some B cells divide into memory cells, which remain circulating in the blood for a long time
• Secondary immune response:

  • responding to previously encountered antigen
  • memory cells recognise antigens and quickly divide into plasma cells, which produce large numbers of the correct antibody to destroy the pathogen
  • pathogens are destroyed before the individual experiences any symptoms
• Active immunity

  • immunity developed after the immune system makes its own antibodies
  • natural: antibodies made after an infection
  • artificial: antibodies made after a vaccination
• Passive immunity

  • immunity acquired by receiving antibodies from another organism
  • natural: antibodies transmitted from mother to baby
  • artificial: antibodies transfused/injected into an individual
• Vaccination

  • introducing dead/attenuated antigens of a pathogen into the body through injection, which stimulates the body to produce an immune response
  • allows the body to develop artificial active immunity
• How vaccination provides immunity

  1. The vaccine, containing antigens, is injected into the blood
  2. This stimulates the primary immune response to produce antibodies against the pathogen
  3. Memory cells are produced, which recognises the antigens
  4. On second exposure, memory cells rapidly divide into plasma cells, which rapidly produce large amounts of antibodies against the pathogen
  5. The pathogen is destroyed before any symptoms are experienced
• Herd immunity

  • when a large proportion of a population is vaccinated and immune to the disease, they cannot transmit the pathogen to others
  • this reduces the chance of non-vaccinated individuals coming into contact with the pathogen
  • as a result, fewer individuals become infected
• Problems with vaccines:

  • people can have a poor response (eg due to malnutrition - little protein to make antibodies)
  • antigentic variation: antigens of a pathogen changes frequently due to genetic mutations
  • antigenic concealment: pathogen hides from immune system
• Antigenic variability

  • if antigens change enough, they would not be recognised by the memory cells produced from vaccination and no immune response is produced
  • as a result, new vaccines have to be made frequently
• HIV structure

  A) Viral envelope

  B) Capsid

  C) Reverse transcriptase

  D) Attachment proteins

  E) 2 RNA strands

  
• HIV replication

  1. Attachment proteins on HIV attach to receptors on a helper T cell
  2. HIV envelope fuses with cell membrane, releasing the capsid into the cell
  3. Reverse transcriptase converts RNA into DNA, which is incorporated into the helper T cell's DNA
  4. DNA is transcribed into viral RNA, then translated to make HIV proteins
  5. The proteins assemble into new HIV particles, which leaves the cell after forming a viral envelope using host cell membrane
• HIV infection progress

  1. Transmission via direct contact with bodily fluids from an infected individual
  2. Acute infection: causes flu-like symptoms
  3. Latency period: no symptoms, HIV replication drops to a low level for a long time
  4. AIDS development: HIV reactivates and destroys helper T cells, individual may develop opportunistic infections eg tuberculosis
• HIV treatment

  • currently incurable
  • antiretroviral therapy can reduce viral replication to low levels such that the infected individual doesn't experience any symptoms or transmit the virus
• HIV destroys helper T cells, so no B cells are activated and divide into plasma cells through mitosis, so no antibodies are produced
• Antibiotics are ineffective against viruses

  • antibiotics target bacterial cell structures eg cell wall, or bacterial enzymes and ribosomes used in metabolic reactions
  • viruses don't have cellular structures, and rely on host cells to carry out metabolic reactions, so antibiotics cannot target and disrupt these reactions
• Monoclonal antibodies

  • identical antibodies produced from a single clone of plasma B cells
  • used in medical diagnosis: bind to specific cells to identify infected cells
  • treatment: target and bind therapeutic drugs to specific infected cells eg cancer cells
• ELISA test

  • uses monoclonal antibodies to detect the presence and quantity of a protein in a sample
  • used to see if a patient has antibodies to a certain antigen (or vice versa)
• ELISA test

  1. Antigen is bound to the bottom of a well in a well plate.
  2. Add antibodies specific to the antigen, which would bind to the antigens. Wash the well to remove any unbound antibodies.
  3. Add a second antibody with an enzyme attached to it, which would bind to the first antibody. Wash the well to remove any unbound antibodies.
  4. Add a substrate that can react with the enzyme to produce a colour change.
  5. Colour change = positive result, colour intensity = quantity