2.4 - Cell Recognition & Immune System

Question 1

What are pathogens?

Answer 1

A microorganism that causes disease.

Question 2

What are the four types of pathogens?

Answer 2

1) Bacteria
2) Viruses
3) Protoctists
4) Fungi

Question 3

What are non-specific defences?

Answer 3

These act quickly to defend the body, but respond in the same way for all pathogens.

Question 4

What are specific defences?

Answer 4

These are slower to defend the body, but produce a specific response for each pathogen.

Question 5

What are the four types of cells that the immune system recognises?

Answer 5

1) Pathogens - The immune system recognises antigens as being foreign and activates cells to destroy the pathogen.

2) Cells from other organisms of the same species - These cells may have different antigens to the body’s own cells and so are identified as being foreign. This can cause the rejection of transplanted organs.

3) Abnormal body cells - Cancerous or infected cells display abnormal antigens that trigger an immune response.

4) Toxins - These are antigen molecules themselves (not cells) and can be recognised by the immune system.

Question 6

What is an antigen?

Answer 6

Antigens are molecules present on the surfce of cell, which trigger an immune response. Antigens include proteins on cell surface membranes.

Question 7

What is a phagocyte?

Answer 7

Phagocytes are a type of white blood cell that engulf and destroy pathogens (phagocytosis). They are found in the blood and body tissues of many organisms.

Question 8

Describe the process of phagocytosis of pathogens

Answer 8

1) Pathogen produces chemicals which attract the phagocyte towards it.

2) The phagocyte has cell surface receptors that attach to chemicals on the surface of the pathogen.

3) Lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the bacterium.

4) The phagosome and lysosome fuse. Lysosome releases lysozymes into the phagosome, where they hydrolyse the bacterium.

5) The hydrolysis products of the bacterium are absorbed by the phagocyte.

Question 9

What is the function of cytotoxic T cells?

Answer 9

Cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens by producing a protein called perforin that makes holes in the cell membrane. Cell membrane becomes permeable and the cell dies as a result.

Question 10

Where do t-lymphocytes mature?

Answer 10

In the thymus gland

Question 11

What are the two forms of T-Lymphocytes?

Answer 11

1) Helper T Cells
2) Cytotoxic T Cells

Question 12

What is the function of cytotoxic T cells?

Answer 12

Cytotoxic T cells directly kill pathogens when stimulated by T Helper Cells

Question 13

What is the function of Helper T Cells?

Answer 13

Helper T Cells release chemicals that activate and stimulate phagocytes and Cytotoxic T Cells. T Helper Cells also activate B Cells which secrete antibodies.

Question 14

Describe the cellular immune response (T Lymphocytes)

Answer 14

1) Pathogens invade body cells or are taken in by phagocytes.

2) The phagocyte places antigens from the pathogen on its cell surface membrane.

3) Helper T Cells with complementary receptors bind to these antigens.

4) This attachment activates the T Cell to divide rapidly by mitosis and form a clone of cells.

5) The cloned cells: developed into memory cells, stimulate phagocytes to engulf pathogens, stimulate B cells to divide and secrete antibodies or activate Cytotoxic T Cells.

Question 15

What is the role of antigen-presenting cells in the cellular response?

Answer 15

An antigen-presenting cell is one of the host’s cells that has been invaded by a pathogen and is dispalcing the antigen on its cell surface membrane. They help to recruit other cells of the immune system to producte a specific immune response. Once the surface receptor of the T cell binds to the specific complementary antigen (on the antigen-presenting cell) it becomes sensitised and starts dividing to produce a clone of cells.

Question 16

What is the role of T cells in stimulating cytotoxic T cells, B cells and phagocytes?

Answer 16

1) Develop into memory cells - these circulate in the body to provide long-term immunity.

2) Stimulate phagocytosis - cloned cells stimulate phagocytes to engulf pathogens.

3) Stimulate division of B cells - cloned cells stimulate B cells to divide and produce antibodies.

4) Activate Cytotoxic T Cells - this allows Tc cells to kill infected cells.

Question 17

What is humoral immunity?

Answer 17

Humoral immunity is so called because it involves antibodies and antibodies are soluble in the blood and tissue fluid (humors).

Question 18

Describe the humoral immune response (B Lymphocytes)

Answer 18

1) A B cell with a complementary antibody binds to the antigens on the pathogen.

2) The B cell engulfs the pathogen and presents its antigen on the cell surface membrane to become an antigen-presenting cell.

3) Activated T helper cells bind to the B cell, causing activation of this B cell (clonal selection)

4) The activated B cell divides by mitosis to give a clone of plasma cells and memory cell clones (clonal expansion).

5) The cloned plasma cells produce and secrete the specific antibody which is complementary to the antigen on the pathogen’s surface. These antibodies attach to antigens on pathogens and destroy them.

6) The memory cells circulate the blood and tissue fluid, ready to divide if the body is re-infected by the same pathogen.

Question 19

What is clonal selection?

Answer 19

The B cell with the correct antibody is selected for cloning (by being activated by a T helper cell)

Question 20

What is clonal expansion?

Answer 20

The division of specific B cells to produce genetically identical clones.

Question 21

What is an antibody?

Answer 21

Antibodies are proteins produced by the immune system in response to foreign substances, such as viruses or bacteria.

Question 22

What is the structure of an antibody?

Answer 22

Have a quarternary structure with two ‘heavy’ polypeptide chains bonded by disulfide bonds to two ‘light’ polypeptide chains.

Question 23

What are the two regions of antibodies?

Answer 23

1) Constant region - This is the same for all antibodies and binds to receptors on cells such as B cells.

2) Variable region - This is different for each antibody as its shape is complementary to a specific antigen. This is the part of the antibody that binds to antigens.

Question 24

What are the three roles of antibodies?

Answer 24

1) Agglutination of pathogens - This involves clumping pathogens together to enable easier phagocytosis.

2) Neutralisation of toxins - This is when antibodies bind to toxins to inactivate them.

3) Preventing pathogens from binding - This is when antibodies bind to pathogens to stop them from infecting body cells.

Question 25

How is an antigen-antibody complex formed?

Answer 25

An antigen and its complementary antibody have complementary molecular shapes. This means that their molecular structures fit into each other. When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens. When this occurs, the two molecules combine to form an antigen-antibody complex.

Question 26

What are the two types of immune response?

Answer 26

1) Primary immune response. 2) Secondary immune response

Question 27

When does the primary immune response take place?

Answer 27

This takes place when the body is exposed to a pathogen for the first time. This response is slow and the infected individual experiences symptoms of the disease.

Question 28

When does the secondary immune response take place?

Answer 28

This takes place when when the body has been exposed to the same pathogen before. This response is much faster and stronger and pathogens are destroyed before any symptoms appear.

Question 29

What is the speed of response of the primary immune response?

Answer 29

Slow

Question 30

Are there any symptoms experienced in the primary immune response?

Answer 30

Yes

Question 31

What cells are activated in the primary immune response?

Answer 31

B and T Cells

Question 32

What is the rate of antibody production in the primary immune response?

Answer 32

Low

Question 33

What number of antibodies are produced in the primary immune response?

Answer 33

Only a few antibodies

Question 34

What is the time between pathogen exposure and antibody production in the primary immune response?

Answer 34

Long (4-7 days)

Question 35

What is the speed of response of the secondary immune response?

Answer 35

Fast

Question 36

What are the symptoms experienced in the secondary immune response?

Answer 36

No

Question 37

What cells are activated in the secondary immune response?

Answer 37

Memory B and T Cells

Question 38

What is the rate of antibody production in the secondary immune response?

Answer 38

High

Question 39

What number of antibodies are produced in the secondary immune response?

Answer 39

Large numbers of antibodies

Question 40

What is the time between pathogen exposure and antibody production in the secondary immune response?

Answer 40

Short (1-3 days)

Question 41

How do vaccines provide protection for individuals and populations against disease?

Answer 41

Vaccination involves the introduction of a pathogen's antigens into the body, usually via injection. This stimulates the body to produce an immune response to the pathogen and in doing so, allows the body to develop artificial active immunity. In order to produce an immune response, vaccines (the injected substance) usually contain antigens. However, they also need to be safe to prevent symptoms of the disease.

Question 42

What can vaccines contain?

Answer 42

1) Dead or inactivated pathogens. 2) Attenuated (weakened) pathogen strains. 3) A harmless version of a toxin. 4) Isolated antigens from a pathogen. 5) Genetically engineered antigens.

Question 43

How does a vaccine provide immunity?

Answer 43

1) The vaccine, containing antigens, is injected into the blood. 2) This stimulates the primary immune response to produce antibodies against the pathogen. 3) Memory cells, capable of recognising these antigens, are produced. 4) On second exposure to this pathogen, memory cells rapidly divide into plasma cells. 5) Plasma cells rapidly produce antibodies against the pathogen. 6) The pathogen is destroyed before any symptoms are experienced.

Question 44

What is herd immunity and how is it achieved?

Answer 44

Herd immunity works if a large proportion of the population is vaccinated against a disease. If most of the population are immune, the chance of someone without immunity coming into contact with the disease is reduced. Herd immunity is the basis of widescale vaccination programs.

Question 45

What is active immunity?

Answer 45

This type of immunity develops when the immune system makes its own antibodies after exposure to a pathogen's antigens. It takes a while to become immune to the disease, but it is long-term protection because memory cells are produced.

Question 46

What is an example of natural active immunity?

Answer 46

Antibodies made after an infection

Question 47

What is an example of artifcial active immunity?

Answer 47

Antibodies made after a vaccination

Question 48

What is passive immunity?

Answer 48

This type of immunity develops when an individual is given antibodies made by a different organism (the individual's immune system does not make these antibodies). This method provides immediate immunity to the disease, but it is short-term protection because the antibodies are broken down and memory cells are not produced.

Question 49

What is an example of natural passive immunity?

Answer 49

Antibodies transmitted from mother to baby.

Question 50

What is an example of artificial passive immunity?

Answer 50

Antibodies transfused or injected into an individual.

Question 51

What is the structure of HIV?

Answer 51

The virus contains the following components: 1) Genetic material - Two single strands of RNA. 2) Enzymes - One of these enzymes is reverse transcriptase, which allows the virus to convert RNA into DNA. 3) Capsid - A layer of protein molecules that surrounds and protects the genetic material. 4) Envelope - An outer layer made up of phospholipids. 5) Glycoproteins - Also known as attachment proteins or envelope proteins, these help the virus to bind to host cells.

Question 52

How is HIV replicated in helper T cells?

Answer 52

1) Attachment proteins on the HIV attach to receptors on a helper T cell. 2) The protein capsid fuses with the T cell membrane and the HIV releases its RNA into the helper T cell. 3) Reverse transcriptase converts this RNA into DNA. 4) The viral DNA is inserted into the helper T cell's genome. 5) The helper T cell's DNA is translated to make viral proteins. 6) The proteins are used to assemble new HIV particles. 7) Fully assembled HIV particles leave the cell in order to infect other cells.

Question 53

How does HIV cause the symptoms of AIDS?

Answer 53

HIV infection progresses through multiple stages that may ultimately lead to the development of AIDS (acquired immunodeficiency syndrome).

Question 54

What are the four stages in the development of HIV to AIDS?

Answer 54

Stage 1) Transmission - HIV is transmitted via direct contact with bodily fluids (e.g. blood or sexual fluids) from an infected individual. Stage 2) Acute infection - Once HIV enters the body, it rapidly replicates, this causes flu-like symptoms for 2 to 4 weeks. Stage 3) Latency period - HIV replication drops to a low level for several years or decades. During this time, the individual usually experiences few or no symptoms. Antiretroviral therapy (ART) can prolong this stage for many years. Stage 4) After some years, HIV reactivates and destroys helper T cells. As the number of T cells in the body drops over time, the immune system begins to fail. At this point, we classify the person as having 'AIDS'.

Question 55

What are the two types of antiretroviral drugs?

Answer 55

1) Reverse transcriptase inhibitors - prevent viral RNA from being copied into DNA for protein synthesis. 2) Protease inhibitors - inhibit proteases used in the synthesis of viral proteins.

Question 56

Why are antibiotics ineffective against viruses?

Answer 56

Antibiotics cannot kill viruses because viruses have different structures and replicate in a different way than bacteria. Antibiotics work by targeting the growth machinery in bacteria (not viruses) to kill or inhibit those particular bacteria.

Question 57

How are monoclonal antibodies used in targeting medication?

Answer 57

Monoclonal antibodies bind to specific cells, bringing therapeutic drugs with them.

Question 58

How are monoclonal antibodies used in medical diagnosis?

Answer 58

1) Monoclonal antibodies bind to specific cell types to identify infected cells.  2) Monoclonal antibodies bind to a pregnancy hormone in home pregnancy testing kits. 3) Monoclonal antibodies can bind to prostate-specific antigens (PSA) to identify prostate cancer in men.

Question 59

What are some ethical issues associated with the use of vaccines?

Answer 59

Who should be responsible for researching and producing vaccines? When testing the effectiveness of vaccines, is it ethically acceptable to have a control group who will not be vaccinated? When stocks of vaccines are low, which groups should be prioritised? If a vaccine is available but expensive, should the NHS be forced to provide it? Should vaccinations be compulsory for children in schools?

Question 60

What are some ethical issues associated with the use of monoclonal antibodies?

Answer 60

Should patients be told if the monoclonal antibodies used for treatment or testing have been produced using transgenic animals? If B cells have been obtained from another person, should they be anonymous donors? Should people with multiple sclerosis be denied treatments involving monoclonal antibodies because of the potentially increased side effects?

Question 61

What are the two types of ELISA test?

Answer 61

1) The direct test - This uses only one antibody. 2) The indirect test - This uses two antibodies.

Question 62

Describe the process of the ELISA test

Answer 62

1) Add the sample(containing the target protein) to a well plate where the target protein can attach to the well. 2) Add the antibody that is specific to the target protein. These antibodies will bind to the target proteins attached to the well. 3) Wash the well to remove any unbound antibodies. 4) Add a second antibody that will bind to the first antibody. These secondary antibodies are attached to an enzyme. 5) Wash the well again to remove any unbound secondary antibodies. 6) Add a solution containing substrate to the well. The enzyme attached to the second antibody will act on the substrate to cause a colour change. The intensity of the colour indicates the quantity of protein present.