2.3 (5) - Cells Recognition and the Immune System

Question 1

What is an antigen?

Answer 1

Any part of an organism (often a protein on a cell’s surface) that is recognised as foreign by our immune system

Question 2

What can our immune system identify by its antigens?

Answer 2

• Pathogens
• Cells from other organisms of the same species
• Cancerous cells
• Toxins

Question 3

Why are transplant organs often take from relatives?

Answer 3

Antigens are genetically controlled - close relatives have more similar antigens

Question 4

What are the 2 types of immune systems?

Answer 4

Specific and non-specific

Question 5

Describe a non-specific immune system

Answer 5

• Response is immediate and the same for all pathogens
• In all animals
• Carries out phagocytosis, using phagocytes
• Physical barriers

Question 6

Describe a specific immune system

Answer 6

• Only in vertebrates
• Response is slower and specific to each pathogen
• Cell-mediated response, using T lymphocytes
• Humoral response, using B lymphocytes

Question 7

What are some physical barriers to infection?

Answer 7

• 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

Question 8

Describe the 5 stages of phagocytosis

Answer 8

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

Question 9

What are T lymphocytes?

Answer 9

• 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

Question 10

Describe the cell-mediated response in 5 stages

Answer 10

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

Question 11

What do cytotoxic T cells do?

Answer 11

• Produce perforin (protein)
• Makes holes in cell-surface membrane
• Results in cell death

Question 12

What are B lymphocytes?

Answer 12

• Required for humoral immunity

- Display and secrete antibodies that can detect antigens and form antigen-antibody complexes

Question 13

Describe the humoral response in 7 stages

Answer 13

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)

Question 14

What are antibodies made up of?

Answer 14

4 polypeptide chains

• 2 heavy chains
• 2 light chains
• Joined by disulfide bonds

Question 15

Describe an antibody’s shape

Answer 15

Form a Y-shaped structure

• The stem = constant region
• The ends of the arms = variable regions that bind to the antigen

Question 16

What are monoclonal antibodies?

Answer 16

• 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

Question 17

How are monoclonal antibodies used in cancer treatment?

Answer 17

• 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

Question 18

Describe how monoclonal antibodies are used in pregnancy tests

Answer 18

• 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

Question 19

What are some ethical issues with using monoclonal antibodies?

Answer 19

• 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

Question 20

What is the primary immune response?

Answer 20

• 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’

Question 21

What are memory cells?

Answer 21

• 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

Question 22

What is the secondary immune response?

Answer 22

• 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

Question 23

What is active immunity?

Answer 23

• 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

Question 24

What is passive immunity?

Answer 24

• 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

Question 25

Describe herd immunity

Answer 25

• If enough people are immune to a pathogen (>90%), the remaining people are often protected
• This is due to a lack of hosts for the pathogen to reproduce in

Question 26

What is antigenic variability?

Answer 26

• Some pathogens have antigens that are constant, so we remain immune to them
• Other pathogens develop new ‘strains’, with different antigens, so our bodies don’t have the memory cells against them
• This can be caused by mutations or the horizontal transfer of plasmids, which contain new genes.
• Infection by new ‘strain’ means that the primary response is needed to detect and respond to the infection

Question 27

What are the 6 components of HIV?

Answer 27

• Attachment proteins
• Lipid envelope
• Capsid
• Matrix
• RNA
• Reverse transcriptase

Question 28

What is the role of attachment proteins in a virus?

Answer 28

Attach to a host cell

Question 29

What is the role of the lipid envelope in a virus?

Answer 29

Derived from the host cell’s membrane, so that it becomes undetectable as a virus

Question 30

What is the role of the capsid in a virus?

Answer 30

Protein layer that encloses HIV’s RNA

Question 31

What is the role of the matrix in a virus?

Answer 31

Protein layer

Question 32

What is the role of the RNA in a virus?

Answer 32

Codes for viral proteins for viral replication

Question 33

What is the role of reverse transcriptase in a virus?

Answer 33

Enzyme for replication (RNA → DNA)

Question 34

Describe the process of HIV replication

Answer 34

1) HIV particles circulate around the bloodstream
2) HIV binds to CD4 glycoprotein of helper T cells (CD4).
3) HIV’s capsid fuses with Th’s cell membrane - releases capsid - it uncoats and releases RNA and reverse transcriptase into Th’s cytoplasm
4) Reverse transcription - RT converts viral RNA into DNA (it can do this because it’s a retrovirus).
5) DNA integrates into the Th cell’s nucleus, DNA is inserted into the host cell’s DNA.
6) Host turns viral DNA into mRNA, mRNA moves out of the Th’s nucleus via a nuclear pore.
7) The mRNA uses the Th cell’s protein synthesis mechanisms to make new HIV particles.
8) New virus particles break away from the Th cell with a bit of its cell membrane, which forms the HIV particle’s lipid envelope.

Question 35

What is AIDS?

Answer 35

HIV kills/interferes with normal functioning of the immune system, leading to AIDS.

Question 36

What does ELISA stand for?

Answer 36

Enzyme Linked Immunosorbent Assay

Question 37

Describe the ELISA test for HIV and AIDS

Answer 37

• It uses antibodies to detect the presence and quality of a protein (antigen).
• Very sensitive, so it can detect the smallest amounts of protein.
• Can be used to detect hepatitis and tuberculosis.
• Can also be useful for detect the amount of drugs in a person.

Question 38

What don’t antibiotics work on viruses?

Answer 38

• There are no metabolic processes or cell structures for antibiotics to disrupt.
• Working within our own body cells - hard for antibiotics to find viruses.
• Viral protein coat means that antibiotics can’t bind to sites.