Cells And The Immune System

Question 1

What are Antigens?

Answer 1

• Molecules (usually proteins) that can generate an immune response when detected by the body
• Found on surface of cells
• Lymphocytes must distinguish between own cells and foreign cells or it would destroy organism’s own tissue

Question 2

Foreign Antigens

Answer 2

• Antigens that aren’t normally found in the body
• It’s these antigens the immune system usually respond to

Question 3

What do Antigens allow the immune system to identify?

Answer 3

Pathogens,Abnormal body cells, Toxins and cells from other individuals of the same species

Question 4

Pathogens

Answer 4

• Organisms that cause disease (bacteria,viruses,fungi)
• All pathogens have antigens on their surface which are identified as foreign by immune system cells which then respond to destroy the pathogen

Question 5

Abnormal Body Cells

Answer 5

Cancerous or pathogen-infected cells have abnormal antigens on their surface, which trigger an immune response

Question 6

Toxins

Answer 6

• These are poisons (also molecules,not cells)
• Some toxins are produced by bacteria
• Immune system can respond to toxins as well as the pathogens that release them
• Toxin itself is an antigen

Question 7

Cells from other individuals of the same species

Answer 7

• When you recieve cells from another person, such as in an organ transplant or blood transfusion, those cells will have some anyigens that are different to your own (unless donor is genetically identical to you)
• Foreign antigens trigger an immune response
• Response leads to rejection of transplanted organs if drugs aren’t taken to suppress the recipient’s immune system
• For blood transfusions (ABO antigens)- if donated blood contains antigens that aren’t recognised, immune response will be generated

Question 8

Non-specific defence mechanism

Answer 8

• Response is immediate and the same for all pathogens
• Includes a physical barrier (skin) and phagocytosis

Question 9

Specific defence mechanism

Answer 9

• Response is slower and specific to each pathogen
• Includes cell mediated response (T lymphocytes) and humoral response (B lymphocytes)

Question 10

How lympocytes recognsise cells belonging to the body?

Answer 10

• The body can identify cells and molecules that belong to itself and foreign cells and molecules
• All cells have proteins on the surface that lymphocytes are able to recognise
• There are millions of types of lymphocytes and each one is able to recognise a different pathogen
• During development lymphocytes in the foetus only encounter self cells and will be able to recognise the proteins on the surface of self cells
• These cells are suppressed or die so that the only remaining lymphocytes respond to foreign material

Question 11

2 types of white blood cell

Answer 11

• Phagocytes- ingest and destroy pathogen by phagocytosis
• Lymphocytes- involved in immune responses

Question 12

What is a phagocyte?

Answer 12

• (macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens)
• Found in blood and tissues
• First cells to respond to an immune system trigger inside body

Question 13

Phagocytosis

Answer 13

• Phagocyte recognises the foreign antigens on a pathogen
• Cytoplasm of phagocyte moves around the pathogen, engulfing it
• Pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte
• Lysosome (an organelle that contains enzymes called lysozymes) fuses with the phagocytic vacuole
• Lysosomes break down the pathogen
• Phagocyte then presents the pathogen’s antigens- stick the antigens on its surface to activate other immune system cells
• The phagocyte is acting as an antigen-presenting cell

Question 14

What are T-cells?

Answer 14

• (T-lymphocyte) is another type of white blood cell
• Has receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes (this activates T-cell)
• Helper T-cells release chemical signals that activate and stimulate phagocytes
• Also activate B-cells which secrete antibodies
• Cytotoxic T-cells kill abnormal and foreign cells
• They do this by producing a protein called perforin which makes holes in cell membranes
• Cell membrane becomes permeable to everything and cell dies

Question 15

Cloned T-cells

Answer 15

• Develop into memory cells
• Activate cytotoxic T-cells
• Stimulate B cells to divide and secrete antibody
• Stimulate phagocytes to engulf pathogens

Question 16

B-Cells

Answer 16

• (B-lymphocytes) are a type of white blood cell
• Covered with antibodies which are proteins that bind to antigens to form an antigen-antibody complex
• Each B-cell has different shaped antibody on its membrane, so different ones bind to different antigens
• Also has receptor proteins on their surface that bind to the signalling molecules released by the helper T-cells
• When antibody on surface of a B-cell meets a complementary shaped antigen, it binds to it
• This, together with substances released from helper T-cells, activates the B-cell
• Known as clonal selection
• Activated B-cell divides into plasma cells by mitosis so cells produced are gentically identical

Question 17

Production of antibodies

Answer 17

• Plasma cells are identical to B-cell (clones)
• They secrete loads of antibodies specific to the antigen- called monoclonal antibodies
• They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes

Question 18

Role of antibodies in the immune system

Answer 18

• Antibody has 2 binding sites, so can bind to 2 pathogens at the same time
• Means that pathogens become clumped together (agglutination)
• This serves as markers that stimulate phagocytes to engulf the bacterial cells to which they are attached (phagocytes bind to antibodies and phagocytose many pathogens at once)
• This process leads to destruction of pathogens carrying this antigen in the body
• Antibodies are proteins
• Specificity of an antibody depends on its variable regions, which form the antigen binding sites
• Each antibody has a variable region with a unique tertiary structure (due to different amino acid sequences) that’s complementary to one specific antigen
• All antibodies havethe same constant regions

Question 19

Cellular Response

Answer 19

T-cells and other immune system cells that they interact with e.g. phagocytes, form the cellular response

Question 20

Humoral Response

Answer 20

B-cells, clonal selection and the production of monoclonal antibodies form the humoral response

Question 21

How does the cellular and humoral response interact with each other?

Answer 21

T-cells help to activate B-cells, and antibodies coat pathogens making it easier for phagocytes to engulf them

Question 22

Plasma Cells

Answer 22

Secrete antibodies (primary immune response)

Question 23

Memory Cells

Answer 23

Live longer and don’t produce antibodies directly (secondary immune response)

Question 24

Primary Response

Answer 24

• When an antigen enters the body for the first time it activates the immune system
• This response is slow because their aren’t many B-cells that can make the antibody needed to bind to it
• Eventually the body will produce enough of the right antibody to overcome the infection
• Meanwhile infected person will show symptoms
• After being exposed to antigen, T-cells and B-cells produce memory cells which remain in the body for a long time
• Memory T-cells remember the specific antigen and will recognise it a second time round
• Memory B-cells record the specific antibodies needed to bind the antigen
• Person is now immune- can respond quickly to second infection

Question 25

Secondary Response

Answer 25

• If same pathogen enters the body again, immune system will produce a quicker, stronger immune response
• Clonal selection happens faster
• Memory B-cells are activated and divide into plasma cells that produce the right antibody to the antigen
• Memory T-cells are activated and divide into the correct type of T-cells to kill the cell carrying the antigen
• This response often gets rid of the pathogen before you begin to show any symptoms

Question 26

Successful Vaccinations and Ethics

Answer 26

• Economically available to immunise most of the population
• Few side effects can discourage individuals
• Producing and transporting vaccine involves specialised equipment (hygiene)
• Must be possible to vaccinate majority to produce herd immunity
• Need staff with appropriate skills
• Herd immunity important as not everyone can be vaccinated (babies immune system isn’t fully functioning)
• Tested on animals before being tested on humans
• Testing vaccines on humans can be risky (may put themselves under risk e.g. unprotected sex)
• People that don’t take vaccine are still protected by herd immunity and people think this is unfair
• If there was an epidemic of a new disease, difficult decisions would have to be made about who would recieve it first

Question 27

Why vaccination may not eliminate disease?

Answer 27

• People may have defective immune systems
• Individuals may develop disease straight after vaccination but before their immunity levels are high enough to prevent it
• Pathogen may mutate frequently (antigenic variability)
• So many varieties of a pathogen so impossible to develop a vaccine that is effective against them all
• Individuals may have objections for religious, ethical or medical reasons
• Certain pathogens hide from body’s immune system, either by concealing themselves in side cells or living in places out of reach (intestines) e.g. cholera

Question 28

Active immunity

Answer 28

• When your immune system makes its own antibodies after being stimulated by an antigen
• Natural= become immune after catching a disease
• Artificial= become immune after you’ve been given a vaccination containing a harmless dose of antigen

Question 29

Passive Immunity

Answer 29

• Type of immunity you get from being given antibodies made by a different organism- your immune system doesn’t produce any antibodies of its own
• Natural= when a baby becomes immune due to the antibodies it recieves from its mother, through the placenta and breast milk
• Artificial= when you become immune after being injected with antibodies from someone else

Question 30

Differences between active and passive immunity

Answer 30

Active Immunity

• Requires exposure to antigen
• Takes a while for protection to develop
• Memory cells are produced
• Protection is long-term because the antibody is produced (after activation of memory cells) in response to complementary antigen being present in the body

Passive Immunity

• Doesn’t require exposure to antigen
• Protection is immediate
• Memory cells aren’t produced
• Protection is short-term because the antibodies given are broken down

Question 31

How do vaccinations work?

Answer 31

• Vaccinations can help avoid symptoms of a disease
• Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing the disease
• This means you become immune without getting symptoms
• Vaccines protect individuals that have them and reduce occurence of disease (herd immunity)
• Vaccines always contain antigens- may be free or attached to a dead or weakened pathogen
• Vaccines may be injected or taken orally (however, orally the vaccine could be broken down by enzymes in the gut or molecules too big to absorbed into the blood)
• Booster vaccines can be given to make sure more memory cells are produced

Question 32

Antigenic Variation

Answer 32

• Pathogens change their surface antigens due to changes in genes of pathogen e.g. influenza virus has different strains each year
• If infected second time, memory cells won’t recognise antigens and a primary response would have to be carried out for the new antigens
• This makes it difficult to develop vaccines

Question 33

Monoclonal Antibodies

Answer 33

• Antibodies produced from a single group of genetically identical B-cells (plasma cells)
• Means they are identical in structure
• Antibodies are very specific because their binding sites have a unique tertiary structure that only an antigen with a complementary shape can fit into
• You can make monoclonal antibodies that bind to anything you want e.g. cell antigen or any other substance and they will only bind to this molecule
• Can be useful for both treating illnesses and in medical diagnosis

Question 34

Direct monoclonal antibody therapy

Answer 34

• Monoclonal antibodies can treat cancer
• Monoclonal antibodies are produced that are specific to antigens on cancer cells
• These antibodies are given to a patient and attach themselves to the receptors on their cancer cells
• They attach to the surface of their cancer cells and block the chemical signals that stimulate their uncontrolled cell growth
• Example is herceptin, which treats breast cancer
• Advantage is since antibodies are not toxic and highly specific, they lead to fewer side effects than other forms of therapy

Question 35

Indirect monoclonal antibody therapy

Answer 35

• Involves attaching a radioactive or cytotoxic drug to the monoclonal antibody
• When the antibody attaches to the cancer cells, it kills them
• Monoclonal antibodies used in this way can be used in smaller doses, as they are targeted on specific sites
• Smaller doses is not only cheaper but reduces side effects the drug might have

Question 36

Pregnancy testing

Answer 36

• Placenta produces a hormone called human chorionic gonadatrophin (hCG) which is found in the mother’s urine
• Monoclonal antibodies present on the test strip are linked to coloured particles
• If hCG is present it binds to these antibodies
• The hCG-antibody-colour complex moves along the strip until it is trapped by a different type of antibody creating a coloured line

Question 37

ELISA test

Answer 37

• Enzyme linked immunosorbant assay
• Uses antibodies to not only detect the presence of a protein in a sample but also the quantity
• It is extremely sensitive and so can detect very small amounts of a molecule
• Imagine trying to find a particular protein e.g. antigen
• First you need to apply the sample to a surface (slide) to which all the antigens in the sample will attach
• Wash the surface several times to remove any unattached antigens
• Add the antibody that is specific to the antigen we are trying to detect and leave the 2 to bind together
• Wash the surface to remove excess antibody
• Add a second antibody that binds with the first antibody (it has an enzyme attached)
• Add the colourless substrate of the enzyme
• Enzyme acts on the substrate to change it into a coloured product
• Amount of antigen present is relative to the intensity of colour that develops
• ELISA is useful at measuring quantity of antigen
• Mere presence of drug is less important than its quantity as many drugs are found naturally in low concentrations
• Indirect ELISA uses a single antibody that is complementary to the antigen but direct ELISA uses 2 different antibodies

Question 38

Ethical issues surrounding monoclonal antibodies

Answer 38

• Animal rights issues- production of monoclonal antibodies involves the use of mice (mice can suffer from trials)
• Testing on humans in drug trials can be dangerous
• Sucessfully used to treat diseases- important that patients have knowledge of risks and benefits of the drugs before giving permission for them to be used (informed consent)
• Must balance advantages and disadvantages

Question 39

HIV structure

Answer 39

• Spherical structure
• Made up of a core containing the genetic material (RNA) and some proteins (including the enzyme reverse transcriptase, which is needed for virus replication)
• Catalyses the production of DNA to RNA
• Presence of this enzyme means HIV belongs to a group called retroviruses
• Has an outer coating of protein called a capsid and a lipid envelope, which is made of membrane stolen from the cell membrane of a previous host cell
• Sticking out from the envelope are attachment proteins that help HIV attach to the host helper T-cell

Question 40

HIV replication

Answer 40

• Being a virus, HIV cannot replicate itself
• Protein on HIV readily binds to receptor molecules (protein CD4) on cell membrane of the host helper T-cell
• Protein capsid fuses with cell-surface membrane and the RNA and enzymes of HIV enter the helper T cell
• HIV reverse transcriptase converts virus’s DNA into RNA
• Newly made DNA is moved into the helper T cell’s nucleus where it is inserted into the cell’s DNA
• HIV DNA in the nucleus creates mRNA using the cell’s enzymes
• mRNA contains the instructions for making new viral proteins and the RNA to go into the new HIV
• mRNA passes out of the nucleus through the nuclear pore and uses the cell’s protein synthesis mechanisms to make HIV particles
• HIV particles break away from the helper T cell with lipid envelope
• Once infected with HIV a person is said to be HIV positive, however replication can go into dormancy

Question 41

How HIV causes symptoms of AIDS

Answer 41

• Human immunodeficiency virus affects the immune system
• HIV specifically attacks helper T cells, which act as host cells for the virus
• Without a sufficient number of helper T cells, immune system can’t stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens
• Memory cells may also become infected and destroyed
• Immune response becomes inadequate and vulnerable to other infections and cancers
• It is these infections that cause death in people with AIDS, and not HIV itself
• People with HIV are classed as having AIDS (acquired immune deficiency syndrome) when their immune system starts to fail and helper T cells drop below a certain level which is critically low

Question 42

Why antibiotics are ineffective against viruses

Answer 42

• Entry of water in bacterial or plant cells would normally cause cell to burst but it doesn’t due to murein in the walls that isn’t easily stretched
• As water enters by osmosis, cell wall resists expansion so water can’t enter
• However, antibiotics inhibit certain enzymes required for synthesis of peptide cross-linkages in bacterial cell walls
• Walls weaken and water can enter naturally, causing cell to burst and die
• Viruses rely on host cells to carry out their metabolic activities as they lack own metabolic pathways and cell structures
• Due to this, antibiotics are ineffective on viruses (viruses also have a protein coat- no sites where antibiotics can work)

Question 43

Controlling HIV infection

Answer 43

• Reduce it’s spread (unprotected sex, infected body fluids, HIV-positive mother to her fetus)
• Not all babies from HIV-positive mothers are born infected with HIV and taking antiviral drugs during pregnancy can reduce the chance of the baby being HIV-positive
• HIV testing before baby is 18 months is inaccurate as baby of HIV-positive may have some HIV antibodies in their blood regardless of whether they are infected or not