Cell Recognition and The Immune System 3.2.4

• What is an antigen?

  A molecule (usually a protein) which causes an immune response
• What is a pathogen?

  A micro-organism which causes disease
• Function of the immune system

  Recognises molecules as self (body cell) or non-self (foreign/ antigen) by the shape of their proteins on their cell-surface membrane
• Defence mechanisms

  
• What is an autoimmune disease?


  A disease in which the body's immune system attacks healthy cells, as it recognises them as foreign
• Phagocytosis


  1. Chemicals are released by the pathogen 2. The phagocytes are attracted to these chemicals ( foreign substances ) and move towards the pathogen3. The phagocyte engulfs the pathogen into a vesicle called a phagosome ( by endocytosis ) 4. Lysosomes found in the phagocyte move towards the vesicle and fuse with the phagosome 5. Lysozymes ( hydrolytic enzymes ) in the lysosomes digests the pathogen 6. The phagocyte displays the important antigens on its cell surface membrane
• Explain the role of antigen-presenting cells (APCs)

  -Macrophage displays antigen from pathogen on its surface -Enhances recognition by TH cells, which cannot directly interface with pathogens/ antigens in body fluid.
• Specific and Non-specific immune response

  Specific= complementary pathogen (B and T lymphocytes) - time lagNon-specific= same for all pathogens (inflammation, phagocytosis) - immediate
• How do lymphocytes recognise cells belonging to the body?

  -In the fetus, lymphocytes constantly colliding with own body cells-Fetus protected from infection -Lymphocytes with receptors complimentary to body cells are suppressed or die-Only remaining lymphocytes are complimentary to foreign material
• What is antigenic variability?

  When the pathogen may mutate frequently so that its antigen changes suddenly rather than gradually
• What causes antigen variability?

  1. Random genetic mutation changes DNA base sequence.2. Results in different sequence of codons on mRNA 3. Different primary structure of antigen = H-bonds, ionic bonds & disulfide bridges form in different places in tertiary structure4. Different shape of antigen
• Explain how antigen variability affects the incidence of disease

  ● Memory cells no longer complementary to antigen = individual not immune = can catch the disease more than once. ● Many varieties of a pathogen = difficult to develop vaccine containing all antigen types.
• Active and passive immunity

  Active= involves direct contact with pathogen and its antigens, stimulating the immune system to produce its own antibodiesPassive= introducing monoclonal antibodies from an outside source
• Contrast active and passive immunity

  
• Examples of active and passive immunity

  ActiveNatural=humoural responseArtificial=vaccinationPassiveNatural=antibodies in breast milkArtificial=anti-venom (injection of antibodies)
• Types of white blood cell (leukocytes)


  -lymphocytes -phagocytes
• Types of lymphocytes

  T-lymphocyte cells = made in bone marrow, matured in thymus glandB-lymphocyte cells = made and matured in bone marrow
• Structure of an antibody

  - made up of 4 polypeptide chains, joined by disulphide bridges- quaternary protein structure

  
• Antibody definition

  Protein with specific binding sites complementary to specific antigens, synthesised and excreted by plasma cells (B-lymphocytes with a large cytoplasm and many organelles)
• How do antibodies destroy pathogens?

  Cause agglutination and subsequent phagocytosisAgglutination= causes antigens to clump together via a network of antigen-antibody complexes, makes it easier for phagocytes to locate and engulf)
• Formation of antigen-antibody complexes

  -Antibody has a primary sequence of amino acids-This gives rise to its specific tertiary structure-Therefore has specific binding sites which are complementary to to specific antigens -To form antigen-antibody complexes
• What are monoclonal antibodies?

  Antibodies produced from a single group of genetically identical B-cells, which all have the same tertiary structure so all specific to the same antigen
• How are monoclonal antibodies artificially produced?

  - mouse immunised by injection of antigen to stimulate production of antibodies - plasma cells isolated from spleen- plasma cells fuse with tumor cells to form a hybridoma - hybridoma produces large amounts of identical antibody molecules
• Name the two types of specific immune response

  -cell mediated -humoural
• Adaptive immune response diagram

  
• Process of Adaptive Immune Response

  1) once in the body, a pathogen is engulfed by a phagocyte by the process of phagocytosis and the phagocyte displays important antigens on its cell-surface membrane (antigen presenting cell)2) a specific helper T-cell with complementary T cell receptor to the antigen being presented, binds to it activating the helper T-cell3) this causes proliferation by mitosis of the helper T-cell to form a large clone4) the clone of T cells go on to carry out specific functions:-a) become cytotoxic/ killer T cells which release perforin to destroy infected cells-b) stimulate phagocytes-c) become more T-helper cells to activate B cells-d) become memory T cells which remain dormant for years waiting for a future infection (have thecomplementary receptor)5) the specific helper T-cell activates a specific B cell with antibodies on its surface complementary to the antigens to its pathogen6) activation of B cell stimulates it to divide by proliferation by mitosis to form a large clone of identical B cells7) majority of these B cells will become plasma cells (secrete antibodies), some become memory B cells
• Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen

  1. Helper T cell binds to the antigen2. This helper T cell stimulates a specific B cell3. B cell divides by proliferation by mitosis4. Forms plasma cells that release antibodies
• What are memory cells?

  -specialised B/T cells produced from a primary immune response-remain in low levels in the blood-can divide rapidly by mitosis is re-exposed to pathogen
• Clinical application of monoclonal antibodies

  -pregnancy tests by detecting HCG hormones in urine-diagnostic procedures such as ELISA test-targeted treatment by attaching drug to antibody
• Direct monoclonal antibody therapy


  1)Monoclonal antibodies are produced that are specific to antigens on cancer cells. 2)These antibodies are given to a patient and attach themselves to receptors on their cancer cells. 3)They attach to the surface of their cancer cells and block the chemical signals that stimulate their uncontrolled growth
• Indirect monoclonal antibody therapy

  1)a radioactive or cytoxic drug is attached to a monoclonal antibody that is specific to antigens2)When the drug is introduced, binds to c cells3 These monoclonal antibodes target specific sites to kill cells4)can be used in small amounts compared to chemotherapy, doesn't affect normal body cells
• Explain the principle of a direct ELISA test

  Detects the presence of a specific antigen1. Monoclonal antibodies bind to bottom of test plate.2. Antigen molecules in sample bind to antibody. Rinse excess.3. Mobile antibody with 'reporter enzyme' attached binds to antigens that are 'fixed' on the monoclonal antibodies. Rinse excess.4. Add substrate for reporter enzyme. Positive result: colour change.
• Explain the principle of an indirect ELISA test

  Detects the presence of an antibody against a specific antigen1. Antigens bind to bottom of test plate.2. Antibodies in sample bind to antigen. Wash away excess.3. Secondary antibody with 'reporter enzyme' attached binds to primary antibodies from the sample.4. Add substrate for reporter enzyme. Positive result: colour change.
• Why is a control done alongside of the ELISA test?

  -to check washing out wells is effective-to check only the presence of the enzyme is causing the colour change
• How to measure concentration using the ELISA test?

  By measuring significance of colour change using a colorimeter to make results quantitative
• uses of ELISA test


  -diagnosing diseases (HIV)-identifying the presence/ concentration of antigen or antibodies
• How does a positive pregnancy test occur?

  1. Person gives a urine sample. If they are pregnant, their urine should contain hCG.2. Reaction site= If hCG is present, it will bind to the free antibody, which has an enzyme attached to it.3. Test site= hCG bound to the free antibody enters the test site.= Here, the hCG-free antibody complex will bind to another fixed antibody using the hCG=This will bring the enzyme (attached to the complex) close to a dye substrate - a reaction occurs, causing a colour change.4. Control site=There is fixed antibody in the control site.=This will bind to any free antibody (not bound to hCG), and cause a colour change here.If a person is not pregnant, the free antibody moves through the test site, and instead binds to the fixed antibody in the control site, causing a colour change here.If a person is pregnant, the hCG-free antibody complex will form in the reaction site, and this complex binds in the test site, causing a colour change here. Some free antibody will still pass to the control site, so you will see two stripes.
• Ethical issues surrounding monoclonal antibodies


  -Animals are used to produce the cells from which monoclonal antibodies are produced-Drug trials against arthritis and leukaemia resulted in multiple organ failure
• Primary and secondary immune response

  -primary is first reaction with effector cells-secondary is with memory cells that exist for many decades unlike effector cells

  
• What is a vaccination?

  The introduction of the appropriate disease antigens into the body to stimulate a primary immune response, without causing symptoms of the disease