5 Cell recognition & The immune system

• Antigens are molecules which when recognised as non-self/foreign by the immune system, can stimulate an immune response and lead to the production of antibodies. They are often proteins on the surface of cells.
• Since proteins have a specific tertiary structure, different proteins can act as specific antigens.
• A pathogen is a disease causing organism. For example, viruses, fungi, bacteria.
• Since antigens are specific, they allow the immune system to identify:

  • Pathogens
  • Cells from other organisms of the same species e.g. organ transplant or blood transfusion
  • Abnormal body cells e.g. cancerous cells/tumours
  • Toxins released from bacteria
• The non-specific immune response includes the phagocytosis of pathogens.
• Phagocytosis:

  1. Phagocyte e.g. macrophage recognises foreign antigens on the pathogen and binds to the antigen
  2. Phagocyte engulfs pathogen by surrounding it with its cell surface membrane/cytoplasm
  3. Pathogen contained in phagosome in cytoplasm of phagocyte
  4. Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes) into the phagosome
  5. Pathogen is hydrolysed/digested
  6. Phagocyte becomes antigen presenting and stimulates specific immune response
• The cellular response is the response of T lymphocytes to a foreign antigen.
• The cellular response:

  1. T lymphocytes recognises antigen presenting cells after phagocytosis
  2. Specific T helper cells with receptor complementary to specific antigen binds to it, becoming activated and dividing rapidly by mitosis to form clones.
• The clones produced by T helper cells:

  • Stimulate B cells for the humoral response
  • Stimulate cytotoxic T cells to kill infected cells by producing perforin
  • Stimulate phagocytes to engulf pathogens by phagocytosis
• The humoral response is the response of B lymphocytes to a foreign antigen.
• The specific immune response includes the cellular response and the humoral response
• The humoral response:

  1. Clonal selection - Specific B cell binds to antigen presenting cell and is stimulated by helper T cells which release cytokines
  2. Clonal expansion - Specific B cell divides rapidly by mitosis to form clones
  3. Some become B plasma cells for the primary immune response so they secrete large amounts of monoclonal antibodies into the blood
  4. Some become B memory cells for the secondary immune response
• The primary response is when an antigen enters the body for the first time
• The role of plasma cels in the primary response:

  • Produces antibodies slower and at a lower concentration because there are not many B cells available that can make the required antibody and T helper cells need to activate B plasma cells to make the antibodies, which takes time
  • Therefore, the infected individual will express symptoms
• The secondary response is when the same antigen enters the body again.
• The role of memory cells in the secondary response:

  • Produces antibodies faster and at a higher concentration because there are both B and T memory cells present and the B memory cells undergo mitosis quicker due to quicker clonal selection
• graph of infection?

  A) Primary response

  B) Secondary response

  
• Antibodies, also called immunoglobins:

  • Are proteins with a quaternary structure
  • Are secreted by B cells e.g. plasma cells and they are produced in response to a specific antigen
  • Bind specifically to antigens forming an antigen-antibody complex
• Antibody diagram:

  A) Antigen

  B) Hinge

  C) Light

  D) Heavy

  E) Disulfide bridge

  F) Constant

  G) Variable

  
• Structure of antibody -> function:

  • Primary structure of protein = sequence of amino acids in polypeptide chain, which determines the folds in the secondary structure as R groups interact, which determines the specific shape of the tertiary structure and position of hydrogen, ionic and disulfide bonds.
  • Quaternary structure consists of 4 polypeptide chains held together by the same bonds, which enables the specifically shaped variable region to form which is a complementary shape to a specific antigen, so an antigen-antibody complex forms.
• Agglutination is when an antibody binds to two or more pathogens at a time, forming an antigen-antibody complex, enabling the pathogens to clump together. Phagocytes can bind to the antibodies and phagocytose many pathogens at once.
• The hinge region in antibodies means they can bind to antigens different distances apart
• What is a vaccination?

  An injection of antigens from dead or weakened pathogens, that stimulates the formation of memory cells.
• A vaccine can lead to symptoms because some of the pathogens might be alive or active, therefore reproduce and release toxins, which can kill cells.
• After vaccination, on reinfection to the same antigen, the secondary response occurs, so antibodies are produced faster and at a higher concentration, leading to the destruction of a pathogen before it can cause harm or symptoms. This is immunity.
• Herd immunity is when a large proportion but not 100% of a population is vaccinated against a disease
• Herd immunity makes it more difficult for the pathogen to spread through the population because:

  • More people are immune so fewer people in the population carry the pathogen
  • Fewer are susceptible so less likely that a non-vaccinated individual will come into contact with an infected person and pass on the disease
• Differences between active and passive immunity:

  A) Initial

  B) No

  C) No

  D) plasma

  E) another

  F) breast milk

  G) Slow

  H) Fast

  I) Long

  J) Short

  
• Vaccines ethical issues:

  • Tested on animals before use on humans
  • Tested on humans and volunteers may put themselves at unnecessary risk of contracting the disease because they think they're fully protected, but the vaccine might not work
  • Can have side effects
  • They are expensive so less money is spent on research and treatments of other diseases
• Antigen variability results in:

  • New vaccine against a disease need to be developed more frequently e.g. influenza
  • Vaccines against a disease may be hard to develop e.g. HIV
  • May experience a disease more than once e.g. common cold
• Effect of antigen variability on disease:

  
  

  1. Change in antigen shape due to a genetic mutation
  2. Antigen not recognised by B memory cells so no plasma cells and antibodies are produced
  3. The infected person is not immune and must re-undergo primary immune response, so antibodies are released slower and at a lower concentration
  4. Disease symptoms are felt.
• Effect of antigen variability on disease prevention:

  1. Change in antigen shape due to a genetic mutation
  2. Existing antibodies with a specific shape are unable to bind to changed antigens and form an antigen-antibody complex
  3. The immune system (memory cells) will not recognise different antigens (strains)
• A suitable vaccine is one that is:

  • Effective - makes memory cells
  • Has no major side effects as they discourage people from being vaccinated
  • Low cost/economically viable
  • Easily produced, transported, stored and administered
• A successful vaccination programme produces a suitable vaccine and provides herd immunity
• In a scatter graph, correlation does not always mean there is a casual relationship, as it could be due to another variable
• Repeatability is when an experiment is repeated using the same method and equipment and obtains the same results
• Validity is the suitability of the investigative procedure to answer the question being asked
• When evaluation a conclusion from a study, mention:

  • Repeatability
  • Validity
  • Reliability
  • If there is a graph, any correlations
  • Potential bias
• A monoclonal antibody is an antibody produced from a single group of genetically identical B cells or plasma cells
• Monoclonal antibodies have an identical structure