Cell recognition & the immune system Flashcards
What is an antigen?
a foreign molecule (protein) that stimulates an immune response which leads to the production of antibodies
How are cells identified by the immune system?
- each type of cell has specific molecules on its surface that identify it
- they’re often proteins which have a specific tertiary structure
What types of cells & molecules can the immune system identify?
- pathogens e.g. viruses, bacteria, fungi
- cells from other organisms of the same species e.g. organ transplants
- abnormal body cells e.g. tumour cells
- toxins released from bacteria
What are the steps of phagocytosis of pathogens?
- phagocyte recognises foreign antigens on pathogen
- phagocyte engulfs pathogen by surrounding it with its cell membrane
- pathogen contained in vesicle (phagosome) in cytoplasm
- lysosome fuses with phagosome & releases lysozymes which are hydrolytic enzymes
- lysozymes hydrolyse pathogens
What does phagocytosis lead to?
the presentation of antigens displaced on the phagocyte cell-surface membrane which stimulates the specific immune response
What is the function of T lymphocytes?
to recognise antigen presenting cells e.g. infected cells, phagocytes presenting antigens, transplanted cells, tumour cells etc.
What is the response of T lymphocytes to a foreign antigen cellular response) ?
- specific helper T cells with complementary receptors bind to antigen on antigen-presenting cell which activates its division by mitosis to form clones
-the clones stimulate: - cytotoxic T cells - kill infected cells by producing perforin
- specific B cells - involve humoral response
- phagocytes - engulf pathogens by phagocytosis
What is the function of B lymphocytes?
they can recognise free antigens in body fluids e.g. blood
What is the response of B lymphocytes to a foreign antigen (humoral response) ?
- clonal selection:
- specific B lymphocyte with complementary receptor binds to antigen
- this is stimulated by helper T cells so divides by mitosis to form clones - some differentiate into B plasma cells which secrete monoclonal antibodies
- some differentiate into B memory cells which remain in blood for secondary immune response
What are antibodies?
quaternary structure proteins secreted by B lymphocytes which specifically bind to antigens forming antigen-antibody complexes
What are the components that make up the structure of an antibody?
- antigen
- antigen binding site
- disulfide bridge
- variable region
- constant region
- light & heavy polypeptide chain
- hinge region
How do antibodies lead to the destruction of pathogens?
- antibodies bind to pathogens forming antigen-antibody complex - they have a specific tertiary structure so binding site is complementary to antigen
- each antibody binds to 2 pathogens at a time causing agglutination of pathogens
- antibodies attract phagocytes
- phagocytes bind to antibodies & phagocytose many pathogens at once
What are the differences between the primary & secondary immune response?
primary - first exposure to antigen:
- antibodies produced slowly at a lower conc
- takes time for specific B plasma cells to be stimulated to produce specific antibodies
- memory cells are produced
secondary - second exposure to antigen
- antibodies produced faster at a higher conc
- B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies
What is a vaccine?
an injection of antigens from dead or weakened pathogens which stimulates formation of memory cells
How do vaccines provide protection to individuals against disease?
- specific B lymphocyte with complementary receptor binds to antigen
- specific T helper cell binds to antigen-presenting cell & stimulates B cell
- B lymphocyte divides by mitosis to form clones
- Some B cells differentiate into B plasma cells which release antibodies
- others differentiate into B memory cells
- on secondary exposure to antigen B memory cells divide by mitosis rapidly to produce B plasma cells
- B plasma cells secrete antibodies faster & at a higher conc
How do vaccines provide protections for populations against disease?
- herd immunity - large proportion of population is vaccinated which reduces spread of pathogen
- large proportion of population is immune so don’t become ill from infection
- fewer infected people pass pathogen on
- unvaccinated people are less likely to come in contact with someone with the disease
What are the differences between active & passive immunity?
- initial exposure to antigen in active immunity & no exposure to antigen in passive
- memory cells are involved in active immunity & aren’t in passive
- antibody produced & secreted from B plasma cells in active immunity but in passive immunity antibody is introduced from another organism e.g. breast milk
- active immunity is slow & takes longer to develop whilst passive immunity is faster acting
- active immunity is long term as antibody can be produced again in response to specific antigen but passive immunity is short term as antibody is hydrolysed
What is the effect of antigen variability on disease & disease prevention?
- antigens on pathogens can change shape due to genetic mutations which creates new strains
- body is no longer immune from vaccine
- B memory cell receptors can’t bind to changed antigen on secondary exposure
- specific antibodies aren’t complementary to changed antigen
What is the structure of a HIV particle?
- lipid envelope
- RNA
- reverse transcriptase
- capsid
- attachment protein
What is the process of HIV replication in helper T cells?
- HIV attachment proteins attach to receptors on helper T cell
- lipid envelope fuses with cell-surface membrane releasing capsid into cell
- capsid uncoats releasing RNA & reverse transcriptase
- reverse transcriptase converts viral RNA to DNA
- viral DNA inserted into helper T cell DNA
- Viral proteins are produced - DNA is transcribed into HIV mRNA which is translated into HIV proteins
- virus particles assemble & are released from the cell by budding
How does HIV cause the symptoms of AIDS?
- HIV infects & kills T Helper cells - so cytotoxic T cells, B cells & phagocytes aren’t stimulated
- immune system deteriorates - more susceptible to infections
- pathogens reproduce, release toxins & damage cells
Why are antibiotics ineffective against viruses?
- viruses don’t have metabolic processes
- viruses don’t have murein cell wall like bacteria
What is a monoclonal antibody?
antibody produced from genetically identical B plasma cells with the same tertiary structure
How are monoclonal antibodies used in medical treatments?
- they have a specific binding site which is complementary to an antigen found on a specific cell type
- therapeutic drug is attached to the antibody
- antibody binds to specific cell forming antigen-antibody complex and delivers the drug
