2.4 Cell recognition and the immune system Flashcards
What is an antigen?
● Foreign molecule / protein / glycoprotein / glycolipid
● That stimulates an immune response leading to production of antibody
How are cells identified by the immune system?
● Each type of cell has specific molecules on its surface (cell-surface membrane / cell wall) that identify it
● Often proteins → have a specific tertiary structure (or glycoproteins / glycolipids)
What types of cells and molecules can the immune system identify?
- Pathogens (disease causing microorganisms) eg. viruses, fungi, bacteria
- Cells from other organisms of the same species (eg. organ transplants)
- Abnormal body cells eg. tumour cells or virus-infected cells
- Toxins (poisons) released by some bacteria
Describe phagocytosis of pathogens (non-specific immune response)
1 Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen
2 Phagocyte engulfs pathogen by surrounding it with its cell membrane
3 Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte
4 Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes)
5 Lysozymes hydrolyse / digest pathogen
Describe the response of T lymphocytes to a foreign antigen (the cellular response)
-T lymphocytes recognise (antigens on surface of) antigen presenting cells eg. infected cells, phagocytes presenting antigens, transplanted cells, tumour cells etc.
-Specific helper T cells with complementary receptors (on cell surface) bind to antigen on antigen-presenting cell → activated and divide by mitosis to form clones which stimulate:
● Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin)
● Specific B cells (humoral response - see below)
● Phagocytes → engulf pathogens by phagocytosis
Describe the response of B lymphocytes to a foreign antigen (the humoral response)
- Clonal selection:
○ Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen
○ This is then stimulated by helper T cells (which releases cytokines)
○ So divides (rapidly) by mitosis to form clones - Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody
- Some differentiate into B memory cells → remain in blood for secondary immune response
What are antibodies?
● Quaternary structure proteins (4 polypeptide chains)
● Secreted by B lymphocytes eg. plasma cells in response to specific antigens
● Bind specifically to antigens forming antigen-antibody complexes
Explain how antibodies lead to the destruction of pathogens
● Antibodies bind to antigens on pathogens forming an antigen-antibody complex
○ Specific tertiary structure so binding site / variable region binds to complementary antigen
● Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens
● Antibodies attract phagocytes
● Phagocytes bind to the antibodies and phagocytose many pathogens at once
Explain 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 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?
● Injection of antigens from attenuated (dead or weakened) pathogens
● Stimulating formation of memory cells
Explain how 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 and stimulates B cell
- B lymphocyte divides by mitosis to form clones
- Some differentiate into B plasma cells which release antibodies
- Some differentiate into B memory cells
- On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells
- These release antibodies faster and at a higher concentration
Explain how vaccines provide protections for populations against disease
● Herd immunity - large proportion of population vaccinated, reducing spread of pathogen
○ Large proportion of population immune so do not become ill from infection
○ Fewer infected people to pass pathogen on / unvaccinated people less likely to come in contact
with someone with disease
Describe the differences between active and passive immunity
Active immunity:
Initial exposure to antigen eg. vaccine or primary infection
Memory cells involved
Antibody produced and secreted by B plasma cells
Slow; takes longer to develop
Long term immunity as antibody can be produced in response to a specific antigen again
Passive Immunity:
No exposure to antigen
No memory cells involved
Antibody introduced from another organism eg. breast milk / across placenta from mother
Faster acting
Short term immunity as antibody hydrolysed (endo/exo/dipeptidases)
Explain the effect of antigen variability on disease and disease prevention
● Antigens on pathogens change shape / tertiary structure due to gene mutations (creating new strains)
● So no longer immune (from vaccine or prior infection)
○ B memory cell receptors cannot bind to / recognise changed antigen on secondary exposure
○ Specific antibodies not complementary / cannot bind to changed antigen
Describe the replication of HIV 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 and reverse transcriptase
- Reverse transcriptase converts viral RNA to DNA
- Viral DNA inserted / incorporated into helper T cell DNA (may remain latent)
- Viral protein / capsid / enzymes are produced
a. DNA transcribed into HIV mRNA
b. HIV mRNA translated into new HIV proteins - Virus particles assembled and released from cell (via budding)
Explain how HIV causes the symptoms of acquired immune deficiency syndrome (AIDS)
● HIV infects and kills helper T cells (host cell) as it multiplies rapidly
○ So T helper cells can’t stimulate cytotoxic T cells, B cells and phagocytes
○ So B plasma cells can’t release as many antibodies for agglutination & destruction of pathogens
● Immune system deteriorates → more susceptible to (opportunistic) infections
● Pathogens reproduce, release toxins and damage cells
Explain why antibiotics are ineffective against viruses
Viruses do not have structures / processes that antibiotics inhibit:
● Viruses do not have metabolic processes (eg. do not make protein) / ribosomes
● Viruses do not have bacterial enzymes / murein cell wall
What is a monoclonal antibody?
● Antibody produced from genetically identical / cloned B lymphocytes / plasma cells
● So have same tertiary structure
Explain how monoclonal antibodies can be used in medical treatments
● Monoclonal antibody has a specific tertiary structure / binding site / variable region
● Complementary to receptor / protein / antigen found only on a specific cell type (eg. cancer cell)
● Therapeutic drug attached to antibody
● Antibody binds to specific cell, forming antigen-antibody complex, delivering drug
Explain how monoclonal antibodies can be used in medical diagnosis
● Monoclonal antibody has a specific tertiary structure / binding site / variable region
● Complementary to specific receptor / protein / antigen associated with diagnosis
● Dye / stain / fluorescent marker attached to antibody
● Antibody binds to receptor / protein / antigen, forming antigen-antibody complex
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens( example 1 Direct ELISA)
Example method 1 (direct ELISA):
1. Attach sample with potential antigens to well
2. Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present
3. Wash well → remove unbound antibodies (to prevent false positive)
4. Add substrate → enzymes create products that cause a colour change (positive result)
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens( example 2 Sandwich ELISA)
- Attach specific monoclonal antibodies to well
- Add sample with potential antigens, then wash well
- Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present
- Wash well → remove unbound antibodies (to prevent false positive)
- Add substrate → enzymes create products that cause a colour change (positive result)
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens( example 2 Indirect ELISA)
- Attach specific antigens to well
- Add sample with potential antibodies, wash well
- Add complementary monoclonal antibodies
with enzymes attached → bind to antibodies if
present - Wash well → remove unbound antibodies
- Add substrate → enzymes create products that
cause a colour change (positive result)
Suggest the purpose of a control well in the ELISA test
● Compare to test to show only enzyme causes colour change
● Compare to test to show all unbound antibodies have been washed away
