Disease and Immunity 10

• What is a pathogen and give examples

  A microorganism that causes disease.

  They can be bacteria, fungi or viruses
• What are the 2 ways pathogens can cause disease?


  • damaging host cells
  • releasing toxins
• Describe the non-specific and specific parts of the immune system

  Non-specific

  • the response is always the same
  • is present from birth
  • is effective against a wide range of pathogens

  Specific

  is slow

  is only effective against specific pathogens

  the response is faster after re-infection (immunological memory)
• what is an antigen?

  a foreign/non-self protein (or glycoprotein) which stimulates an immune response. They can be on the surface of a cell/virus or freely circulating in blood (e.g. toxins). All cells/viruses have antigens which are used in cell recognition, they are specific to each organism so immune system can identify foreign or non-self molecules
• What do antigens allow the immune system to identify?

  • pathogens (e.g. HIV)
  • cells from other organisms of the same species (e.g. from transplants)
  • abnormal body cells (e.g. cancer cells)
  • toxins (normally proteins)
• define phagocytosis


  when a phagocyte engulfs a foreign substance, such as a bacterium, encloses it within a vesicle which then fuses with lysosomes in the cytoplasm
• lysosomes contain hydrolytic enzymes. What is the function of these?


  they are powerful, digestive enzymes which hydrolyse cell components e.g. biological molecules
• sequence of events in phagocytosis

  • phagocyte recognises and binds to an antigen on the pathogen
  • phagocyte engulfs pathogen
  • pathogen is enclosed in a vesicle- phagosome
  • lysosomes fuse with the vesicle- phagolysosome
  • lysosomes contain hydrolytic enzymes (lysozymes) that damage bacterial cell walls
  • these digest the pathogen, by hydrolysis of its molecules

  the phagocyte places antigens from the pathogen on its cell surface membrane, becoming an antigen presenting cell
• the two types of specific immune response?

  humoral (b-cells/ outside cells)

  cellular (t-cells/ inside cells)
• The cellular response involves T lymphocytes which recognise non-self antigens presented on the surface of other body cells, called antigen presenting cells
• What can antigen presenting cells be?

  • cells that are infected by a virus
  • a phagocyte which has just engulfed and hydrolysed a pathogen
  • an abnormal cell such as a cell which has become cancerous
• T cells only respond to antigens presented by other body cells, and the response involves the destruction of infected ( or even abnormal) body cells, it doesn't involve antibodies
• summary of the role of T cells in cellular immunity

  • a specific helper T cell binds to an antigen on an antigen presenting cell ( the helper T cell has receptors which have a complementary shape to the antigens)
  • this activates the Th cell to divide rapidly by mitosis
  • to produce more Th cells and memory cells
  • Th cells release cytokines which stimulate specific cytotoxic T cells to divide by mitosis
  • cytotoxic T cells release perforin which destroys virus infected cells
• What does the humoral response involve?


  Involves B cells and antibody production. It’s effective against pathogens which are circulating in the bloodstream
• Summary of the humoral response?


  • Receptors on a specific B cell bind to an antigen
  • the B cell is activated by cytokines released by a specific Th cell
  • the B cell divides by mitosis to produce cells which differentiate into plasma cells and memory B cells (clonal selection)
  • plasma cells release and produce antibodies
  • memory cells remain in the body and divide to produce plasma cells which release more antibodies more rapidly following second exposure to the same antigen
• The primary immune response


  The initial response following the body’s first exposure to an antigen. It takes a few days for the body to activate a specific B cell, which then divides by mitosis to produce plasma cells which then release antibodies . During this time disease symptoms develop
• Secondary immune response 


  Much faster and occurs when the body is exposed to the same antigen again. Memory cells divide rapidly to produce plasma cells. These plasma cells release more antibodies more rapidly. The pathogen is usually destroyed before symptons develop
• 3 organelles you would expect to find in large quantities in plasma cells


  • mitochondria- to produce ATP from respiration to provide energy to form peptide bonds between amino acids for synthesis of proteins, antibodies-proteins
  • golgi body- to process the proteins and transport them out of the cell via exocytosis
  • rough ER- ribosomes for protein synthesis, can't be ribosomes free in cytoplasm as the proteins produced are needed outside cell
• Compare the cellular response and humoral response to a pathogen


  both produce memory cells

  involve lymphocytes

  involve mitosis

  involves T helper cells
• Contrast cellular and humoral response

  cellular- effective against damaged, infected cells

  humoral- effective against pathogens circulating in the bloodstream

  cellular- cytotoxic T cells

  humoral- B cells

  humoral- antibodies

  cellular- no antibodies
• describe antibodies


  • proteins synthesised by plasma cells
  • they have a variable region-different amino acid sequence
  • and a constant region- same amino acid sequence
  • they have a specific tertiary structure, complementary to a particular antigen
  • they bind to specific antigens forming an antigen-antibody complex
  • they neutralise pathogens by clumping them together- called agglutination
  • phagocytes can then engulf the pathogens and digest them
• what level of protein structure do antibodies show?

  quaternary
• compare antigens and antibodies


  both have a specific structure

  ab are proteins and ag are usually proteins-(could be glycoprotein)

  both involved in the immune response
• Differences between antigens and antibodies


  Ag-trigger an immune response

  Ab- complementary to an antigen, binds to and destroys it so part of the immune reponse
• What are monoclonal antibodies?

  antibodies produced from the same B cell/ plasma cell. They have all have the same tertiary structure and will therefore bind to one specific antigen.
• How can monoclonal antibodies be made?

  By injecting mice with the antigen you wish to identify

  • the mouse produces specific plasma cells by the normal immune response
  • the specific plasma cells are extracted and purified
  • they are then fused with myeloma cells to produce hybridoma cells
  • these cells produce and release monoclonal antibodies specific for the antigen
• uses of monoclonal antibodies

  • medical diagnosis- to screen patients for specific infections
  • targeted drug treatments- cancer drugs can be attached to monoclonal antibodies specific for antigens on cancer cells
  • drug testing- used to identify a specific drug in a urine or blood sample
  • pregnancy testing- used to detect a specific pregnancy hormone
  • COVID testing- used to detect antigens on coronavirus
• The ELISA test (enzyme linked immunosorbent assay)

  Uses monoclonal antibodies to detect the presence and amount of a specific protein in a sample. The test is highly sensitive and can detect the presence of a substance even in very small amounts
• Antigen detection process

  1. Container coated with monoclonal antibodies specific for the antigen being screened for

  2. Sample to be tested (e.g. blood/urine) is added to the container

  3. If the specific antigen is present it will bind to the antibodies

  4. A second monoclonal antibody is added that binds to another site on the antigens

  5. The second Ab has an enzyme attached

  6. The sample is then washed

  7. The substrate for the enzyme is then added

  8. The substrate is hydrolysed by the enzyme on the second antibody

  9. This produces a colour change, indicating a positive result (ie the antigen was present in the original sample)
• Why is it important to wash after the second antibody is added?


  to remove unbound 2nd antibody in order to prevent a false positive result
• The test can be used to detect the amount of antigen present in the sample. Suggest how

  the more antigen present, the more antigen-antibody complexes will form and more e-s complexes formed so more coloured product
• Compare active and passive immunity

  Passive- antibodies acquired from an outside source

  active- immune system is stimulated to produce antibodies

  p- immunity is immediate

  a-immunity not immediate, memory cells need to be produced

  p-effect is short-lived as the antibodies will be broken down and not replaced

  a-effect is long-term because memory cells are produced

  p-foetus receives antibodies from the mother across the placenta

  a-can be acquired naturally by becoming infected with a disease/pathogen or can be acquired artificially by vaccination
• what is vaccination?

  the introduction of an antigen into an organism, with the intention of making them immune to a specific disease.
• Describe sequence of events following vaccination

  • vaccine contains specific antigens, these could be isolated antigens, or antigens on killed or attenuated pathogens
  • the antigen is displayed on an antigen presenting cell
  • specific helper T cell binds to the antigen
  • cytokines released by helper T cells activate a specific B cell
  • B cell divides by mitosis to produce plasma cells and memory cells
  • on 2nd exposure to same antigen, memory cells divide to produce plasma cells so more antibodies are released, more rapidly
  • pathogen is destroyed before symptoms develop
• What makes a vaccination programme successful?

  • vaccine must have few, if any, side effects
  • vaccine must be economically available in quantities sufficient to immunise the entire vulnerable population
  • must be easily stored and transported
  • must have means of administration
• herd immunity


  if around 90% of the population are vaccinated against a particular disease, this creates herd immunity which prevents the spread of the disease. The pathogen will be killed quickly in vaccinated people. People who are not vaccinated have a very low chance of coming into contact with another unvaccinated person
• Why vaccination does not eliminate a disease?


  • some individuals may have a defective immune system
  • the pathogen may mutate quickly and change its antigens
  • there may be many varieties of the same organism, each with a different antigen
• Reasons for and against vaccination


  for-less people develop the disease

  for- less spread of pathogen to individuals who cannot be vaccinated

  against-involves animal testing

  may have side effects

  clinical trials in humans (may occur in developing countries where a target disease is prevalent)

  compulsory vaccination required for herd immunity

  balancing costs- at what point is a vaccination programme discontinued?
• reasons for and against monoclonal antibodies


  for- provide new, successful treatments for specific conditions e.g. cancer and diabetes

  against- involves animal testing e.g. may need to induce cancer in mice

  there have been multiple deaths when treating multiple sclerosis

  drug testing in humans sometimes goes wrong
• antigenic variability


  diseases such as chicken pox are only developed once as memory cells produced following a primary immune response allow a rapid secondary response. However, influenza virus and some other pathogens have many different strains, the antigens on the surface of each strain are different- antigenic variability. Existing memory cells do not recognise the new antigens, so a primary response is initiated and symptoms of the infection develop