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
e
Describe phagocytosis of pathogens (non-specific immune response)
- Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen
- Phagocyte engulfs pathogen by surrounding it with its cell membrane
- Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte
- Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes)
- Lysozymes hydrolyse / digest pathoge
Phagcytosis leads to …
Phagocytosis leads to presentation of antigens where antigens are displayed on the phagocyte cell-surface
membrane, stimulating the specific immune response (cellular and humoral response)
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)
B lymphocytes can recognise free antigens eg. in blood or tissues, not just antigen presenting cells.
1. 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
2. Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody
3. 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
Struc of antibody
2x antigen binding sites
2x variable regions
2x constant region
2x light chain
2x heavy chain
1x disulphide bridge between heavy and light chain
2x disulphide bridge between 2 heavy chains
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 onc
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
6.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 diseas
Describe the differences between active and passive immunity
Initial exposure to antigen eg.
vaccine or primary infection
Vs No exposure to antigen
Memory cells involved vs No memory cells involved
Antibody produced and secreted
by B plasma cells
Vs Antibody introduced from another organism eg.
breast milk / across placenta from mother
Slow; takes longer to develops Faster acting
Long term immunity as antibody can be produced
in response to a specific antigen again vs
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 struc of a HIV particle
Attachment protein on lipid envelope
Capsid surrounds RNA and reverse transcriptase
Describe the replication of HIV in helper T cells
1.HIV attachment proteins attach to receptors on helper T cell
2. Lipid envelope fuses with cell-surface membrane, releasing capsid into cell
3. Capsid uncoats, releasing RNA and reverse transcriptase
4. Reverse transcriptase converts viral RNA to DNA
5. Viral DNA inserted / incorporated into helper T cell DNA (may remain latent)
6. Viral protein / capsid / enzymes are produced
a. DNA transcribed into HIV mRNA
b. HIV mRNA translated into new HIV proteins
7. 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 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