Cell recognition and the immune system

• What is an antigen

  ~Cell surface molecule which stimulate immune response

  ~Usually glycoprotein, sometimes glycolipid or polysaccaride

  ~Immune system recognises as self and non self = enables identification of cells from other organisms of same species, pathogens, toxins and abnormal body cells
• How does phagocytosis destroy pathogens

  ~Phagocyte moves towards pathogen via chemotaxis

  ~Phagocyte engulfs pathogen via endocytosis to form a phagosome

  ~Phagosome fuses with lysosome

  ~Lysozymes digest pathogen

  ~Phagocyte absorbs the products from pathogen hydrolysis
• Explain the role of antigen-presenting cells

  ~Macrophage displays antigen from pathogen on its surface

  ~Enhances recognition by Th cells, which cannot directly interface with pathogens/antigens in body fluid
• Give two differences between specific and nonspecific immune responses

  Nonspecific

  ~same for all pathogens

  ~immediate

  Specific

  ~complementary pathogen

  ~time lag
• Name the two types of specific immune response

  ~Cell mediated

  ~Humoral
• Outline the process of the cell mediated response

  ~Complementary Th lymphocytes bind to foreign antigen on APC

  ~Release cytokines that can stimulate

  ~Clonal expansion of complementary Th cells: become memory cells or trigger humoral response

  ~Clonal expansion of cytotoxic T cells: secrete enzyme perforin to destroy infected cells
• Outline the process of the humoral response

  ~Complementary Th lymphocytes bind to foreign antigen on antigen-presenting T cells

  ~Release cytokines that stimulate clonal expansion of complementary B lymphocytes

  ~B cells differentiate into plasma cells

  ~Plasms cells secrete antibodies with complementary variable region to antigen
• What is an antibody

  ~Proteins secreted by plasma cells

  ~Quaternary structure: two light chains held together by disulfide bridges, two longer heavy chains

  ~Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen

  ~The rest of the region is known as the constant region
• How do antibodies lead to the destruction of a pathogen

  Formation of antigen-antibody complex results in agglutination, which enhances phagocytosis
• What are monoclonal antibodies


  Antibodies produced from a single clone of B cells
• What are memory cells

  ~Specialised Th/B cells produced from primary immune response

  ~Remain in low levels in the blood

  ~Can divide very rapidly by mitosis if organism encounters the same pathogen again
• What causes antigen variability

  ~Random genetic mutation changes DNA base sequence

  ~Results in different sequence of codons on mRNA

  ~Different primary structure of antigens = H-bonds, ionic bonds and disulfide bridges form in different places in tertiary structure

  ~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
• Compare passive and active immunity. Give examples of both types


  ~Both involve antibodies

  ~Can both be natural or artificial

  ~Passive natural: antibodies in breast milk/across placenta

  ~Passive artificial: anti-venom, needle stick injections

  ~Active natural: humoral response to infection

  ~Active artificial: vaccination
• Explain the principles of vaccination

  ~Vaccine contains dead/inactive form of a pathogen or antigen

  ~Triggers primary immune response

  ~Memory cells are produced and remain in the bloodstream, so secondary response is rapid and produces higher concentration of antibodies

  ~Pathogen is destroyed before it causes symptoms
• What is herd immunity

  ~Vaccinating large proportion of population reduces available carriers of the pathogen

  ~Protects individuals who have not been vaccinated
• Suggest some ethical issues surrounding the use of vaccines

  ~Production may involve use of animals

  ~Potentially dangerous side-effects

  ~Clinical tests may be fatal

  ~Compulsory vs opt-out
• Describe the structure of HIV

  ~ Genetic material and viral enzymes surrounded by capsid

  ~Surrounded by viral envelope derived from host cell membrane

  ~GP120 attachment proteins on surface
• How does HIV results in the symptoms of AIDS


  ~Attachment proteins bind to complementary CD4 receptor on Th cells

  ~HIV particles replicate inside Th cells, killing or damaging them

  ~AIDS develops when there are too few Th cells for the immune system to function

  ~Individuals cannot destroy other pathogens and suffer from secondary diseases/infections
• Why are antibiotics ineffective against viruses

  ~Antibiotics often work by damaging murein cell walls to cause osmotic lysis. Viruses have no cell wall

  ~Viruses replicate inside host cells = difficult to destroy them without damaging normal body cells
• Suggest the clinical applications of monoclonal antibodies

  ~Pregnancy tests by detecting HCG hormones in urine

  ~Diagnostic procedures

  ~Targeted treatment by attaching drug to antibody so that it only binds to cells with abnormal antigen
• Explain the principle of a direct ELISA test


  ~Detects presence of a specific antigen

  ~Monoclonal antibodies bind to bottom of test plate

  ~Antigen molecules in sample bind to antibody. Rinse excess

  ~Mobile antibody with reporter enzyme attached binds to antigens that are fixed on the monoclonal antibodies. Rinse excess

  ~Add substrate for reporter enzyme. Positive result: colour change
• Explain the principle of an indirect ELISA test

  ~Detects presence of an antibody against a specific pathogen

  ~Antigens bind to bottom of test plate

  ~Antibodies in sample bind to antigen. Wash away excess

  ~Secondary antibody with receptor enzyme attached binds to primary antibodies from the sample

  ~Add substrate for reporter enzyme. Positive result: colour change
• Suggest some ethical issues surrounding the uses of monoclonal antibodies


  ~Production involves animals

  ~Drug trials against arthritis and leukemia resulted in multiple organ failure