Topic 5 - Cell recognition and the immune system Flashcards
(32 cards)
Antigen
Molecules which, when recognised as non self/foreign by the immune system, can stimulate an immune response and lead to the production of antibodies
The specificity of antigens allows the immune system to identify…
- Pathogens e.g. viruses, fungi, bacteria
- Cells from other organisms of the same species e.g. organ transplant
- Abnormal body cells e.g. cancerous cells
- Toxins released from bacteria
Phagocytosis of pathogens - non specific immune response
- Phagocyte recognises foreign antigens on the pathogen and binds to the antigen
- Phagocyte engulfs pathogen by surrounding it with its cell surface membrane / cytoplasm
- Pathogen contained in vacuole/vesicle/phagosome in cytoplasm of phagocyte
- Lysosome fuses with phagosome and releases lysosomes (hydrolytic enzymes) into the phagosome
- These hydrolyse/digest the pathogen
- Phagocyte becomes antigen presenting cell and stimulates specific immune response
Cell mediated response
- T lymphocytes recognises antigen presenting cells after phagocytosis
- Specific T helper cell with receptor complementary to specific antigen binds to it, becoming activated and dividing rapidly by mitosis to form clones which:
a) Stimulate B cells for the humoral response
b) Stimulate cytotoxic T cells to kill infected cells by producing perforin
c) Stimulate phagocytes to engulf pathogens by phagocytosis
The humoral response
- Clonal selection:
a) specific B cell binds to antigen presenting cell and is stimulating by helper T cells which releases cytokines
b) divides rapidly by mitosis to form clones (clonal expansion) - Some become B plasma cells for the primary immune response - secrete large amounts of monoclonal antibody into blood
- Some become B memory cells for the secondary immune response
Primary response
- Produces antibodies slower and at a lower concentration because:
- Not many B cells available that can make the required antibody
- T helpers need to activate B plasma cells to make antibodies (takes time)
- So infected individual will express symptoms
Secondary response
- Produces antibodies faster and at a higher concentration because
- B and T memory cells present
- B memory cells undergo mitosis quicker / quicker clonal selection
Antibodies
- Quaternary structure protein
- Secreted by B lymphocytes
- Binds specifically to antigens forming an antigen-antibody complex
Describe and explain how the structure of an antibody relates to its function
- Primary structure of protein = sequence of amino acids in a polypeptide chain:
- Determines the folding the secondary structure as R groups interact
- Determines the specific shape of the tertiary structure and position of hydrogen, ion and disulphide bonds
- Quaternary structure is comprised of 4 polypeptide chains held together by hydrogen, ionic and disulphide bonds:
- Enables the specific shaped variable region (binding site) to form which is a complementary shape to a specific antigen
- Enables antigen-antibody complex to form
How do antibodies work to destroy pathogens e.g. bacterial cells
- Binds to two pathogens at a time (at variable region/binding site) forming an antigen-antibody complex
- Enables antibodies to clump the pathogens together - agglutination
- Phagocytes bind to the antibodies and phagocytose many pathogens at once
Note: The hinge region means an antibody can bind to antigens/pathogens different distances apart.
What is a vaccination?
- injection of antigens
- From attenuated (dead or weakened) pathogens
- Stimulates the formation of memory cells
- A vaccine can lead to symptoms because some of the pathogens might be alive therefore, the pathogen could reproduce and release toxins, which can kill cells
The use of vaccines to provide protection for individuals against disease
- Normal immune response but the important part is that memory cells are produced
- On reinfection / secondary exposure to the same antigen, the secondary response therefore produces antibodies faster and at a higher concentration
- Leading to the destruction of a pathogen before it can cause harm = immunity
The use of vaccine to provide protection for populations against disease (herd immunity)
- Large proportion but not 100% of population 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
- Few 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
Active immunity:
- Initial exposure to antigen e.g. vaccine or primary infection
- Memory cells involved
- Antibody is produces and secreted by (B) plasma cells
- Slow; takes time to develop
- Long term immunity –> antibody can be produced un response to specific antigen again
Passive immunity:
- No exposure to anitgen
- No memory cells involved
- Antibody introduced into body from another organism e.g. breast milk/across placenta from mother
- Fast acting
- Short term immunity (antibody broken down)
Ethical issues associated with the use of vaccines
- Tested on animals before use on humans –> animals have a CNS so feel pain
- Tested on humans –> volunteers may put themselves at unnecessary risk of contracting the disease because they think they’re fully protected e.g. HIV so have unprotected sex
- Can have side effects
- Expensive - less money spent on research and treatments of other diseases
Antigen variability is often an explanation for why…
- New vaccines against a disease need to be developed more frequently e.g. influenza
- Vaccines against a disease may be hard to develop or cant be developed in the first place e.g. HIV
- May experience a disease more than once e.g. common cold
Explain the effect of antigen variability on disease
- Change in antigen shape (due to genetic mutation)
- Not recognised by B memory cell - no plasma cells/antibodies
- Not immune
- must re-undergo primary immune response –> slower/relsease lower concentration of antibodies
- Disease symptoms felt
Explain the effect of antigen variability on disease prevention (vaccines)
- Change in antigen shape (due to genetic mutation
- Existing antibodies with a specific shape unable to bind to changed antigens / form antigen-antibody complex
- Immune system i.e. memory cells won’t recognise different antigens (strain)
Evaluate methodology, evidence and data relating to the use of vaccinations
A successful vaccination programme:
- Produce suitable vaccine:
- Effective - make memory cells
- No major side effects –> side effect discourage individuals from being vaccinated
- Low cost/economically viable
- Provides herd immunity
The use of monoclonal antibodies
- Monoclonal antibody = antibody produced from a single group of genetically identical (clones) B cells / plasma cells
- Identical structure
- Binds to specific complimentary antigen
- Have a binding site with a specific tertiary structure
- Only one complementary antigen will fit
Why are monoclonal antibodies useful in medicine
- Only bind to specific target molecules
- Antibodies have a specific tertiary structure that’s complementary to a specific antigen which can bind/fit to the antibody
Monoclonal antibodies: targeting medication to specific cell types by attaching a therapeutic drug to an antibody
Example: Cancer cells
- Monoclonal antibodies made to be complementary to antigens specific to cancer cells –> cancer cells are abnormal body cells with different antigens (tumour markers)
- Anti-cancer drug attached to antibody
- Antibody binds / attaches to cancer cells (forming antigen-antibody complex)
- Delivers attached anti-cancer drug directly to specific cancer cells so drug accumulates –> fewer side effects e.g. fewer normal body cells killed
Exam question example: some cancer cells have a receptor protein in their cell-surface membrane that binds to a hormone called growth factor. This stimulates the cancer cells to divide. Scientists have produced a monoclonal antibody that stops this stimulation. Use your knowledge of monoclonal antibodies to suggest how this antibody stops the growth of a tumour (3 marks)
- Antibody has specific tertiary structure / binding site / variable region
- Complementary (shape / fit) to receptor protein / GF / binds to receptor protein
- Prevents GF binding (to receptor)
Monoclonal antibodies: medical diagnosis
Example: pregnancy test
- Pregnant women have the hormone hCG in their urine
- Urine test strip has 3 parts with 3 different antibodies:
- Application area, position 1: antibodies complementary to hCG (bound to a blue coloured bead)
- Middle, position 2: antibodies complementary to hCG-antibody complex
- End position 3: antibodies complementary to antibody without hCG attached
- If pregnant:
- hCG binds to antibodies in application area = hCG antibody complex
- Travels up test strip, binds to antibodies at position 2 = blue line
If not pregnant:
- No hCG in urine so hCG doesn’t bind to antibodies in application area so doesn’t bind to antibodies in position 2 = no blue line
- Bind to antibodies at position 3 –> blue line = control
