Cell Recognition & the Immune system Flashcards
Cells revision
What is an antigen?
- Cell surface molecule which stimulate immune response
- Usually (glyco)protein, sometimes (glyco)lipid or polysaccharide
- Immune system recognises as “self” or “non-self” = enables identification of cells from other organisms of same species, pathogens, toxins & 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 (phagolysosome)
- Lysozymes digest pathogen
- Phagocyte absorbs products from pathogen hydrolysis
Explain role of antigen-presenting cells (APCs)
Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis)
Enhances recognition by Th cells, which cannot directly interface with pathogens/ antigens in body fluid
2 Diferences between specific and non-specific immune responses
Non-specific (inflammation, phagocytosis) = same for all pathogens
Specific (B & T lymphocytes) = complementary pathogen
Non-specific = immediate
Specific = time lag
Outline process of cell-mediated response
- Complementary Th lymphocytes bind to foreign antigen on APC
- Release cytokines that stimulate:
a) clonal expansion of complementary Th cells (rapid mitosis): become memory cells or trigger humoral response
b) clonal expansion of cytotoxic T cells (Tc): secrete enzyme perforin to destroy infected cells
Outline process of humoral response
- Complementary Th lymphocytes bind to foreign antigen on antigen-presenting T cells
- Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes
- B cells differentiate into plasma cells
- Plasma cells secrete antibodies with complementary variable region to antigen
What is an antibody?
proteins secreted by plasma cells
Quaternary structure: 2 ‘light chains’ held together by disulfide bridges, 2 longer ‘heavy chains’
Binding sites on variable region of light chains have specific tertiary structure complementary to antigen
Rest of molecule known as constant region
How do antibodies lead to destruction of 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 blood
- Can divide very rapid by mitosis if organisms encounters same pathogen again
Contrast primary & secondary immune response
secondary response:
- faster rate of antibody production
- shorter time lag between exposure & antibody production
- Higher conc. of antibodies
- Antibody level remains higher after secondary response
- Pathogen usually destroyed before any symptoms
What causes antigen variability?
- Random genetic mutation changes DNA base sequence
- Results in different sequence of codons on mRNA
- Different primary structure of antigen = H-bonds, ionic bonds & disulfide bridges form in different places in tertiary strucutre
- Different shape of antigen
Explain how antigen variability affects incidence of disease
- Memory cells no longer complementary to antigen = individual not immune = can catch disease more than once
- Many varieties of pathogen = difficult to develop vaccine containing all antigen types
Compare passive and active immunity.
- 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 principles of vaccination
- Vaccine contains dead/ inactive form of pathogen or antigen
- Triggers primary immune response
- Memory cells are produced & remain in bloodstream, so secondary response is rapid & produces higher conc. of antibodies
- Pathogen is destroyed before it causes symptoms
What is herd immunity?
Vaccinating large proportion of population reduces available carriers of pathogen.
Protects individuals who have not been vaccinated (e.g. those with weak immune system)
Suggest ethical issues surrounding use of vaccines
- Production may involve use of animals
- Potentially dangerous side-effects
- Clinical tests may be fatal
- Compulsory vs opt-out
Describe structure of HIV
- Genetic material (2x RNA) & viral enzymes (integrase & reverse transcriptase) surrounded by capsid
- Surrounded by viral envelope derived from host cell membrane
- GP120 attachment proteins on surface
How does HIV result in 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 & suffer from secondary diseases/ infections
Why are antibiotics ineffective against viruses?
Antibiotics often work by damaging murein cell walls to cause osmosis lysis. Viruses have no cell wall
Viruses replicate inside host cells = difficult to destroy them without damaging normal body cells.
Suggest clinical applications of monoclonal antibodies
- Pregnancy tests by detecting HCG hormones in urine.
- Diagnostic procedures (e.g. ELISA test)
- Targeted treatment by attaching drug to antibody so that it only binds to cells with abnormal antigen (e.g. cancer cells due to specificity of tertiary structure of binding site)
Explain principle of a direct ELISA test
detects presence of 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 monoclonal antibodies. Rinse excess
- Add substrate for reporter enzyme. Positive result: colour change
Explain principle of an indirect ELISA test
detects presence of an antibody against specific antigen
- Antigen bind to bottom of test plate
- Antibodies in sample bind to antigen. Wash away excess.
- Secondary antibody with ‘reporter enzyme’ attached binds to primary antibodies from samples
- Add substrate for reporter enzyme. Positive result: colour change
Suggest some ethical issues surrounding use of monoclonal antibodies
- Production involves animals
- Drug trials against arthritis & leukaemia resulted in multiple organ failure
