Cells

Question 1

What are antigens?

Answer 1

Antigens are molecules (usually proteins) that are usually found on the surface of cells, including body cells.

Question 2

What are foreign antigens?

Answer 2

Antigens that aren’t normally found in the body are referred to as foreign antigens - it is these antigens that the immune system will usually respond to.

Question 3

What do antigens allow the immune system to identify?

Answer 3

• Pathogens
• Abnormal body cells
• Toxins
• Cells from other individuals of the same species

Question 4

What are pathogens?

Answer 4

Organisms that cause disease (bacteria,fungi). All pathogens have antigens on their surface and are identified as foreign by the immune system.

Question 5

What are abnormal body cells?

Answer 5

Cancerous or pathogen infected cells have antigens on the surface which trigger an immune response.

Question 6

What are toxins?

Answer 6

Poisons (molecules, not cells) can be produced by bacteria, the immune system responds to toxins as well as the pathogen that releases them.

Question 7

What are cells from other individuals of the same species?

Answer 7

Cells received from another person (e.g. organ transplant/blood transfusion) the cells will have antigens that are different to your own, the foreign antigens trigger an immune response which leads to rejection if drugs aren’t used to suppress the recipients immune system.

Question 8

What are the four main stages of the immune system?

Answer 8

• Phagocytosis
• T-Cells
• B-Cells
• Antibody production

Question 9

What is a phagocyte?

Answer 9

A phagocyte is (e.g. a macrophage) is a type of white blood cell that carries out phagocytosis, which is the engulfment of pathogens. Phagocytes are the first cells to response to an immune system trigger inside the body.

Question 10

Explain how phagocytosis works.

Answer 10

1) A phagocyte has receptor proteins on their surface that recognise the antigens on a pathogen as foreign. The phagocyte binds to the surface of the pathogen.
2) Once attached the cell surface membrane of the phagocyte extends out and around the pathogen, engulfing it and trapping the pathogen within a phagocytic vacuole (phagosome). This part is known as endocytosis.
3) Lysosomes fuse with the phagocytic vacuole and releases lysozymes to digest the pathogen (e.g. hydrolysis of the bacterial cell walls). Soluble products from the break down are absorbed into the cytoplasm.
4) The phagocyte then presents the pathogen antigens on its surface to activate other immune system cells. The phagocyte is acting as an antigen presenting cell.

Question 11

What are T-Cells and the two types?

Answer 11

T-Cells (T-lymphocytes) is another type of white blood cell. It has receptor proteins on its surface that bind to complimentary antigens presented to it by phagocytosis. This activates the T-cell, they divide in number by mitosis. Different T-Cells respond in different ways.
The two types of T-Cells are Helper T-Cells and Cytotoxic T-Cells (Killer T-Cells).

Question 12

What do Helper T-Cells do?

Answer 12

• Release chemical signals (cytokines) that activate and stimulate phagocytes and cytotoxic T-Cells which kill abnormal and foreign cells.
• Helper T-Cells also activate B-Cells (B-lymphocytes) which secrete antibodies.

Question 13

What do Cytotxic T-Cells do?

Answer 13

• They are activated by Helper T-Cells and they kill abnormal or foreign cells.

Question 14

What are B-Cells?

Answer 14

• B-Cells (B-lymphocytes) are a type of white blood cell. They’re membranes are covered with antibodies - proteins that bind to antigens to form antigen-antibody complex. Each B-Cell has a different shaped antibody, therefore binds to different antigens, they have a complimentary shape.
• When bound with an antigen this activates the B-Cell (along with substances released from Helper T-Cells). This is called clonal selection. The activated B-Cells divide by mitosis (clonal expansion) into plasma cells. Plasma cells are identical to the B-Cells (clones produced by mitosis). They secrete loads of antibodies specific to the antigen - therefore are monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes. Antibodies have tow binding sites; this means pathogens become clumped together which is called agglutinations. Phagocytes then bind to the antibodies and phagocytose many pathogens at once

Question 15

What is the structure of an antibody?

Answer 15

Antibodies are proteins - chains of amino acids. The specificity of an antibody depends on its variable region which form the antigen binding sites. Each antibody has a variable region with an unique tertiary structure that’s complimentary to one specific antigen. All antibodies have the same constant region.
The parts of an antibody are: Variable regions, Hinge regions, Constant regions, Light chains, Disulfide bridges and Heavy chains.

Question 16

What are the two immune responses?

Answer 16

• Cellular response - The T-Cells and other immune system cells that they interact with e.g. phagocytes.
• Humoral response - B-Cells, clonal selection and the production of monoclonal antibodies form the humoral response.

Question 17

What is the primary response?

Answer 17

• When an antigen enters the body for the first time it activates the immune system.
• Slow response - aren’t many B-Cells that can make the antibody needed to bind to it.
• Whilst the body is producing enough of the right antibody, the infected person shows symptoms of the disease.
• After being exposed to an antigen both T-Cells and B-Cells produce memory cells, that remain in the body for a long time.
• Memory T-Cells remember this specific antigen.
• Memory B-Cells record the specific antibodies needed to bind to the antigen.
• The person is now immune - has ability to respond quickly to a second infection.

Question 18

What is the secondary response?

Answer 18

• When the pathogen enters the body again.
• A quicker, stronger immune response.
• Clonal selections happen faster.
• Memory B-Cells are activated and divide into plasma cells to produce the right antibody.
• Memory T-Cells are activated and divide into the correct type of T-Cells to kill the cell carrying the antigen.
  Secondary response often gets rid of the pathogen before you show symptoms.

Question 19

What are the two types of immunity?

Answer 19

• Active immunity
• Passive immunity

Question 20

What is active immunity?

Answer 20

The immune system makes own antibodies against and antigen either;
- Natural - become immune after catching a disease.
- Artificial - become immune after a vaccine.

Question 21

What is passive immunity?

Answer 21

Immunity from being given antibodies made by a different organism;
- Natural - baby becomes immune due to antibodies it receives from placenta and breast milk.
- Artificial - become immune after being injected with antibodies from someone else.

Question 22

What are the differences between active and passive immunity?

Answer 22

• Active immunity are exposed to an antigen, passive does not require exposure to an antigen.
• Active takes a while for the protection to develop, in passive immunity protection is immediate.
• Active immunity = memory cells are produced, in passive immunity memory cells are not produced.
• Active immunity is long term because antibodies are produced in response to complimentary antigen being present in the body whereas passive immunity is short term as antibodies given will be broken down.

Question 23

What are vaccines and what do they do?

Answer 23

Vaccinations avoid you from suffering from a pathogen.
Antigens in vaccines cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without getting any symptoms.
Vaccines protect individuals that have them, therefore reducing the occurence of the disease, those not vaccinated are also less likely to catch the disease-herd immunity.
- If more people are immune the likelihood of passing antigen on decreases.
Vaccines usually contain antigens which are either free floating or attached to a dead or attenuated (weakened) pathogen.
Vaccines can be injected or taken orally. The disadvantage of taken orally is that it could be broken down by enzymes or the molecules of the vaccine could be too large to be absorbed into the blood.
Booster vaccines can be given later on to make sure more memory cells are produced.

Question 24

What are some ethical issues with vaccines?

Answer 24

• Animal testing - all vaccines are tested on animals.
• Animal based substances may be used to produce a vaccine.
• Testing on humans can be risky - volunteers may put themselves at risk of contracting the disease because they think they are protected but the vaccine hasn’t worked.
• Side effects - some people do not want to take the risk.
• If there was an epidemic of a new disease - difficult to decide who would be the first to receive the vaccine.

Question 25

What is antigenic variation?

Answer 25

Some pathogens can change their surface antigens, this is called antigenic variation. This is due to changes in the genes of a pathogen. This means that when you're infected for a second time the memory cells produced from the first infection will not recognise the different antigens. So the immune system has to start from scratch and carry out the primary response against the new antigens. This takes time and hence you get ill again. Antigenic variation makes it difficult to develop vaccines against some pathogens. Examples of pathogens that show antigenic variation include HIV and the influenza virus.

Question 26

Explain how antigenic variation effects the influenza virus and why there are different vaccines for the flu.

Answer 26

The influenza vaccine changes every year, because the antigens on the surface changes regularly, forming new strains of the virus. Memory cells produced from the vaccination with one strain of the flu will not recognise other strains with different antigens; the strains are immunologically distinct (structure has changed). Every year there are different strains of the influenza virus circulating in the population, so a different vaccine has to be developed. New vaccines are developed, and one is chosen every year that is most effective against the recently circulating influenza viruses, governments and health authorities the implement a programme of vaccination using the most suitable vaccine.

Question 27

What are monoclonal antibodies?

Answer 27

Monoclonal antibodies are antibodies produced from a single group of genetically identical B-Cells (plasma cells), therefore identical in structure. Antibodies are very specific because their binding sites have a unique tertiary structure that only an antigen with a complimentary shape can fit into. Monoclonal antibodies can be made that will bind to anything you want e.g. a cell antigen or other substances and they will only bind to this molecule. This is useful for both treating illnesses and in medical diagnosis.

Question 28

How can monoclonal antibodies be used against cancer cells?

Answer 28

- Cancer cells have antigens called tumour markers that are not found on normal body cells. Monoclonal antibodies can be made that will bind to the tumour markers (complimentary shape). - Anti-cancer drugs can also be attached to the antibodies, so when they come into contact with the cancer cells they will bind to the tumour markers. This means the drug will only accumulate in the body where there are cancer cells. Therefore, side effects of an antibody-based drug are lower than other drugs.

Question 29

How can monoclonal antibodies be used in pregnancy tests?

Answer 29

- Pregnancy tests detect the hormone HCG that's found in the urine of pregnant women. - The application area contains antibodies that are complimentary to the HCG protein, bound to a coloured bead. - When urine is applied to the application area any HCG will bind to the antibody on the beads, forming an antigen-antibody complex. - The urine moves up the stick to the test, carrying any beads. - The test strip contains antibodies to HCG that are stuck in place. - If there is HCG present the test strip turns blue because the immobilised antibody binds to any HCG - concentrating the HCG antibody complex with blue beads attached. If no HCG is present , the beads will pass through the test area without binding to anything, so will not turn blue.

Question 30

What are ELISA tests and how do they work?

Answer 30

The enzyme-linked immunosorbent assay (ELISA) allows you to see if a patient has any antibodies to a certain antigen or any antigen to a certain antibody. For example, it can be used in medical diagnosis to test for pathogenic infections (HIV infection), or for allergies (e.g. to nuts or lactose). In an ELISA test, an antibody is used with an enzyme attached. This enzyme can react with a substrate to produce a coloured product. This causes the solution in the reaction vessel to change colour. If there is a colour change, it demonstrates that the antigen or antibody of interest is present in the sample being tested. In some types of ELISA, the quantity of this antigen/antibody can be worked out from the intensity of the colour change.

Question 31

Describe the role of antibodies in producing a positive result in an ELISA test. (4 marks).

Answer 31

- First antibody binds to antigen. - Second antibody with enzyme attached is added. - Second antibody attaches to first antibody. - Substrate is added and colour changes.

Question 32

What is a direct ELISA?

Answer 32

A direct ELISA uses a single antibody that is complimentary to the antigen you're testing for. Antigens from a patient sample are bound to the inside of a well in a well plate. A detection antibody (with an attached enzyme) that is complimentary to the antigen of interest is added. If the antigen is present in the patient sample, it will be immobilised on the inside of the well and the detection antibody will bind to it. The well is the washed out to remove any unbound antibody and a substrate solution is added. If the detection antibody is present, the enzyme reacts with the substrate to give a colour change. This is a positive result for presence of the antigen.

Question 33

What is an indirect ELISA?

Answer 33

Indirect ELISA uses two different antibodies. It can be used to see if a patient possesses antibodies to HIV. 1) HIV antigen is bound to the bottom of a well in a well plate. 2) A sample of the patients blood plasma, which might contain several different antibodies, is added to the well. If there are any HIV-specific antibodies in the plasma (i.e. antibodies against HIV) these will bind to the HIV antigen stuck to the bottom of the well. The well is then washed out to remove any unbound antibodies. 3) A secondary antibody, that has a specific enzyme attached to it, is added to the well. This secondary antibody can bind to the HIV-specific antibody (which is also called the primary antibody). The well is washed out again to remove any unbound secondary antibody. If there's no primary antibody in the sample, all of the secondary antibody will be washed away because there will be nothing for it to bind to. 4) A solution is added to the well. This solution contains a substrate, which is able to react with the enzyme attached to the secondary antibody and produce a coloured product. If the solution changes colour, it indicates that the patient has HIV-specific antibodies in their blood and is infected with HIV.

Question 34

What happens when you have HIV?

Answer 34

HIV (human immunodeficiency virus) affects the immune system, specifically the T-Cells. It leads to AIDS (acquired immune deficiency syndrome) which is a condition where the immune system deteriorates and eventually fails. Making people very vulnerable to infections. This occurs when T-Cells reach a critically low level. HIV infects and eventually kills helper T-Cells.

Question 35

Why are helper T-Cells so important in the immune system?

Answer 35

They send chemical signals that activate phagocytes, cytotoxic T-Cells and B-Cells.

Question 36

Without T-Cells what will happen?

Answer 36

The immune system will not produce an effective response to infections because other immune system cells won't behave how they should.

Question 37

HIV infection. What happens in the initial infection and the latency period?

Answer 37

Initial infection - HIV rapidly replicates and the infected person experiences flu-like symptoms. Latency period - After the initial period HIV replication drops, during this latency period the infected person won't experience symptoms (this can last for years).

Question 38

Symptoms of AIDS.

Answer 38

People with HIV are classed as having AIDS when symptoms of their failing immune system start to appear or their helper T-Cell count drops below a certain level. Length of time between HIV infection and development of AIDS varies between individuals, but without treatment (antiviral drugs) this is usually around 10 years. Symptoms start with: - Minor infections of mucus membranes and recurring respiratory infections. - As AIDS progresses the number of immune system cells decreases further. Patients become more susceptible to more serious infections (chronic diarrhoea, severe bacterial infections and tuberculosis). - During the later stages of AIDS patients have very low number of immune system cells and can develop a range of serious infections (toxoplasmosis of the brain and candidiasis of the respiratory system). It is these serious infections that kill an AIDS patient, not the HIV itself.

Question 39

What are factors that may affect progression of HIV to AIDS?

Answer 39

- The strain of HIV - Age - Access to healthcare - Existing infections

Question 40

HIV structure.

Answer 40

- Spherical structure. - Core containing the genetic material (RNA) and some proteins (including reverse transcriptase), which is needed for viral replication. -Outer coating of protein called a capsid and an extra outer layer called an envelope (made of the membrane from previous host cell). - Sticking out of the envolope are attachment proteins.

Question 41

How does HIV replicate?

Answer 41

1) The attachment protein attaches to a receptor molecule on the cell membrane of the host helper T-Cell. 2) The capsid is released into the cell, where it uncoats and releases the genetic material (RNA) into the cell's cytoplasm. 3) Inside the cell, reverse transcriptase is used to make a complimentary strand of DNA from the viral RNA template. 4) From this, double stranded DNA is made and inserted into the human DNA. 5) Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA. (DNA is transcribed into HIV mRNA which is the translated into HIV proteins). 6) The viral proteins are assembled into new viruses "bud" from the cell (which ruptures and dies) and HIV goes on to infect other cells.

Question 42

How do antibiotics work?

Answer 42

- Antibiotics kill bacteria by interfering with their metabolic reactions. - They are designed to target the bacterial enzymes and ribosomes, which are different to human enzymes/ribosomes.

Question 43

Why do antibiotics not work on viruses?

Answer 43

Because viruses do not have their own enzymes and ribosomes - they use the ones in the host cells to replicate therefore antibiotics can't inhibit them because they don't target the human process.

Question 44

What do antiviral drugs do and what is one example?

Answer 44

Most antiviral drugs are designed to target the few virus-specific enzymes that exist. Example: HIV uses reverse transcriptase to replicate. Human cells do not use this enzymes, so drugs are designed to inhibit it without affecting the host cell. Thses drugs are called reverse-transcriptase inhibitors. Antiviral drugs only slow down the progression of HIV infection to AIDS.

Question 45

How to control HIV infection?

Answer 45

There is no cure or vaccine for HIV. The best way to control HIV infection is by reducing it's spread. HIV can be spread by: - Unprotected sexual intercourse. - Through infected bodily fluids (blood from sharing needles). - From HIV positive mother to her fetus (antiviral drugs during pregnancy can reduce the chance of the baby being HIV positive).