2.3 (5) - Cells Recognition and the Immune System Flashcards
Defence mechanisms Phagocytosis T lymphocytes and cell-mediated immunity B lymphocytes and humoral immunity Antibodies Vaccination HIV
What is an antigen?
Any part of an organism (often a protein on a cell’s surface) that is recognised as foreign by our immune system
What can our immune system identify by its antigens?
- Pathogens
- Cells from other organisms of the same species
- Cancerous cells
- Toxins
Why are transplant organs often take from relatives?
Antigens are genetically controlled - close relatives have more similar antigens
What are the 2 types of immune systems?
Specific and non-specific
Describe a non-specific immune system
- Response is immediate and the same for all pathogens
- In all animals
- Carries out phagocytosis, using phagocytes
- Physical barriers
Describe a specific immune system
- Only in vertebrates
- Response is slower and specific to each pathogen
- Cell-mediated response, using T lymphocytes
- Humoral response, using B lymphocytes
What are some physical barriers to infection?
- Skin - physical barrier that pathogens find difficult to penetrate
- Epithelial cells in airways
- Mucus - epithelial layers inside the body produce mucus that pathogens stick to, which immobilises them
- Stomach acid - denatures enzymes or coats proteins of most pathogens that enter the stomach
Describe the 5 stages of phagocytosis
1) Pathogen gives off chemicals, so the phagocyte is attracted towards it PHAGOCYTE DETECTS AND MOVES TOWARDS CHEMICALS RELEASED FROM THE PATHOGEN ALONG A CONCENTRATION GRADIENT
2) Pathogen attaches to the phagocyte
3) The phagocyte engulfs the pathogen. It migrates to the phagosome in a vesicle
4) Lysozyme hydrolyses (break down) the pathogens in the phagosome
5) The phagocyte absorbs the products of hydrolysis
What are T lymphocytes?
- Required for cell-mediated immunity
- Have receptor proteins that, on each cell, can detect one specific antigen
- Occur in 2 forms:
1) Helper T-cells
2) Cytotoxic T-cells
Describe the cell-mediated response in 5 stages
1) Pathogens invade cells OR are taken in by phagocytes
2) The phagocyte places antigens from the pathogen on its cell membrane
3) Receptors on a specific helper T cell fix exactly onto these antigens
4) This attachment activates the T cell to divide rapidly my mitosis and form a clone of genetically-identical cells
5) The activated, cloned T-cells can:
- divide by mitosis to produce helper-T cell clones (memory cells)
- release cytokines, to activate cytotoxic-T cells
- activate B cells to produce antibodies
- stimulate phagocytosis
What do cytotoxic T cells do?
- Produce perforin (protein)
- Makes holes in cell-surface membrane
- Results in cell death
What are B lymphocytes?
- Required for humoral immunity
- Display and secrete antibodies that can detect antigens and form antigen-antibody complexes
Describe the humoral response in 7 stages
1) The surface antigens of an invading pathogen are taken up by a B-cell
2) The B-cell processes the antigens and presents them on its surface
3) Helper T-cells attach to the processed antigens on the B-cell, activating the B-cell
4) The B-cell is now activated to divide by mitosis to give a clone of plasma cells
5) The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface
6) The antibody attaches to antigens on the pathogens and stops disease by:
- sticking cells together in large clumps (agglutination) and targeting them for destruction by phagocytosis
- preventing viruses and bacteria from infecting cells
- binding to free toxin proteins
7) Some B-cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies (secondary immune response)
What are antibodies made up of?
4 polypeptide chains
- 2 heavy chains
- 2 light chains
- Joined by disulfide bonds
Describe an antibody’s shape
Form a Y-shaped structure
- The stem = constant region
- The ends of the arms = variable regions that bind to the antigen
What are monoclonal antibodies?
- Antibodies produced from a single group of genetically-identical B-cells (or plasma cells) specific to 1 type of antigen
- They are all identical in structure because:
1) they have the same primary structure, as they are coded for by the same genes
2) so, they have the same secondary and tertiary structures, because of being coded for by the same genes
How are monoclonal antibodies used in cancer treatment?
- Cancer cells have tumour markers
- Monoclonal antibodies can be produced to bind to these tumour markers. Anti-cancer drugs can be attached to the antibodies
- Therefore, the drugs will only be released where antibody binding occurs (ie, at cancer cells)
- This importantly reduces side effects because the drugs will only accumulate at specific cells
Describe how monoclonal antibodies are used in pregnancy tests
- The placenta in a pregnant woman produces hCG. Can be found in the mother’s urine
- Monoclonal antibodies are immobilised in coloured beads on a test strip
- When urine is applied, the hCG will bind to the antibodies to form antigen-antibody complexes
- The hCG-antibody-colour complex moves along the strip until it is trapped by a different type of antibody. The complexes accumulate to produce a coloured line to confirm pregnancy
What are some ethical issues with using monoclonal antibodies?
- Production involves mice to produce monoclonal antibodies, but to do this, they need to produce tumour cells
- Negative results involved deaths in treating multiple sclerosis
What is the primary immune response?
- Happens when a new pathogen first invades
- Relatively few initial specific T and B-cells
- Because we start with less T and B-cells, relatively few clones are produced
- Disease’s symptoms become visible
- Once primary infection has been detected and responded to the person, they has become ‘immune’
What are memory cells?
- Some of the B and helper T-cells differentiate into memory cells
- They remain in the blood for a number of years, so they can keep responding to pathogens
What is the secondary immune response?
- Happens when the same pathogen infects for a second time
- Memory cells are present and ready to respond to a second infection (weeks, months or years later)
- Much faster response to re-infection:
- Memory T-cells divide into cytotoxic T-cells
- Memory B-cells divide into plasma cells
- Many more T and B-cells produced, therefore there is a much stronger response
- Pathogens are destroyed before it can cause symptoms
What is active immunity?
- AKA vaccination
- Vaccines contain dead/weakened pathogens
- They do not cause disease but they do contain the antigens necessary to evoke a primary immune response
- If the person later becomes infected by the live pathogen, the secondary response will occur
What is passive immunity?
- Occurs naturally (immunity passed from mother → baby)
- And artificially (injecting antibodies):
- Used when someone has already been infected (or is likely to become infected) with a pathogen
- Antibodies extracted from blood plasma of an infected person
- The antibodies assist the body’s normal immune response
- Does not itself lead to long-term immunity
