P2 The Immune System

Question 1

What are pathogens, and what are the main types of pathogen?

Answer 1

• Microorganisms that cause communicable diseases.
• Bacteria, viruses, fungi and protoctista.

Question 2

What diseases do bacteria cause?

Answer 2

1. Tuberculosis (affects lungs)
2. Bacterial meningitis (affects brain)
3. Ring rot (kills potatoes and tomatoes)

Question 3

What diseases do viruses cause?

Answer 3

1. Tobacco mosaic virus (affects plants leaves)
2. Influenza
3. HIV (causes AIDS)

Question 4

What diseases do fungi cause?

Answer 4

1. Athletes foot
2. Cattle ringworm
3. Black sigatoka (kills banana plants)

Question 5

What diseases do protoctista cause?

Answer 5

1. Malaria (affects red blood cells)
2. Late blight (causes potatoes and tomatoes to decay)

Question 6

What are the methods of direct transmission of pathogens in humans?

Answer 6

1. Direct contact
2. Droplets eg. when an infected person sneezes

Question 7

What are the methods of indirect transmission of pathogens in humans?

Answer 7

1. Ingestion (contaminated food/water)
2. Vectors (small organisms not affected by pathogens, water, air or spores)

Question 8

What are the factors affecting pathogen transmission in humans?

Answer 8

1. Population density
2. Few trained healthcare workers
3. Insufficient health education
4. Climate change (a warmer temperature means more pathogens/vectors can survive)

Question 9

What is the method of direct transmission in plants?

Answer 9

Direct contact

Question 10

What are the methods of indirect transmission in plants?

Answer 10

1. Contaminated soil
2. Vectors (water, air, humans and small organisms)

Question 11

What are the factors affecting pathogen transmission in plants?

Answer 11

1. Growing susceptible crops
2. Overcrowding
3. Damp, warm environment

Question 12

What are plants physical defences against pathogens?

Answer 12

1. Barriers - such as the thick, waxy cuticle and the thick cellulose cell wall in palisade cells.
2. Plants can close their stomata to prevent pathogens entering.
3. When a pathogen invades , the plant produces callose which is deposited between the cell surface membrane and the cell wall to reinforce the cell wall and prevent pathogens entering the cell.
4. If a leaf is infected, the plant can detach the leaf (leaf abscission).

Question 13

What are plants chemical defences against pathogens?

Answer 13

Plants release hydrolytic enzymes to break down the cell wall of invading pathogens.

Question 14

What are humans primary, non-specific defences?

Answer 14

1. Skin (physical barrier)
2. Mucous membranes - line the airways, gut and reproductive system. Contain goblet cells that secrete mucus, the mucus traps pathogens and contains enzymes and white blood cells that kill bacteria.
3. Expulsive reflexes (coughing and sneezing)
4. Blood clotting - if the skin is wounded blood clots near the wound, forming a scab and preventing pathogens entering. Cells beneath divide by mitosis to heal the wound.
5. Inflammation - if pathogens enter through a wound, the site of the wound becomes inflamed (blood vessels around the wound dilate and increase in permeability), causing fluid to enter the tissue (swelling) and allows white blood cells to enter the tissue and kill the pathogens.

Question 15

How to identify phagocytes and lymphocytes in a blood smear?

Answer 15

Neutrophils (a phagocyte) have a lobed nucleus, whereas lymphocytes have a round nucleus.

Question 16

What is the role of lymphocytes?

Answer 16

Recognise foreign/non-self cells and trigger a specific immune response (due to antigens on the cell surface membrane of non-self cells).

Question 17

What is an antigen?

Answer 17

A protein in the cell surface membrane that triggers an immune response.

Question 18

How do lymphocytes recognise antigens?

Answer 18

• All lymphocytes have complementary antigen receptors on their surface.
• When the correct lymphocyte binds to the antigen, the specific immune response is triggered.

Question 19

What is the role of phagocytes?

Answer 19

• Phagocytes recognise non-self cells (pathogens).
• They have complementary receptors to all pathogens that enter the body (as all pathogens have the same molecular pattern on their surface).
• This triggers a non-specific immune response.

Question 20

Describe phagocytosis:

Answer 20

1. The phagocyte is attracted to chemicals released by the pathogen and moves along a concentration gradient.
2. The phagocyte binds to the pathogen and recognises it as non-self.
3. The phagocyte engulfs the pathogen to form a phagosome.
4. Lysosomes in the phagocyte fuse with the phagosome, forming a phagolysosome.
5. The lysosomes release lysozymes which hydrolyse the pathogen.

Question 21

What is the role of cytokines (non-specific immune response)?

Answer 21

When a phagocyte engulfs a pathogen, it releases cytokines which attract more phagocytes to the site of infection, making sure all pathogens are destroyed.

Question 22

What is the role of opsonins (non-specific immune response)?

Answer 22

• Opsonins bind to antigens on a pathogen and prevent the pathogen from repelling phagocytes.
• Phagocytes have complementary receptors to opsonin, meaning the phagocyte binds to opsonin, that is bound to the pathogen.
• The phagocyte can then engulf the pathogen, and hydrolyse it.

Question 23

What are the two types of lymphocyte?

Answer 23

1. B lymphocytes/B cells - produced and mature in the bone marrow.
2. T lymphocyte/T cells - produced in the bone marrow but mature in the thymus.

Question 24

How are B cells involved in the specific immune response?

Answer 24

1. Newly matured (naive) B cells with complementary receptors bind to the antigen on the pathogen.
2. The B cell then engulfs the pathogen and breaks it down.
3. The B cell preserves the antigens, processes them and then presents them on the surface of the cell - an antigen presenting cell.
4. Helper T cells with complementary receptors to the processed antigen bind to the antigen and release interleukins which stimulate the B cell to clone itself.
5. The cloned B cells then differentiate into plasma cells or memory B cells.

Question 25

What is the role of plasma cells?

Answer 25

• Plasma cells only survive for a few days, they secrete proteins called antibodies.
• Antibodies have 4 polypeptide chains, 2 shorter (light chains) and 2 longer (heavy chains). The individual chains are held together by disulfide bridges.
• Antibodies are made complementary to the antigen that triggers the B cell response. They contain 2 antigen binding sites - the variable region of the antibody. B cell receptors have a very similar structure to antibodies.

Question 26

What is an antibody?

Answer 26

A protein produced by naive B cells in response to the presence of a specific antigen.

Question 27

What are monoclonal antibodies?

Answer 27

Antibodies with the same tertiary structure, produced from cloned B cells.

Question 28

How do antibodies cause agglutination in the specific immune response?

Answer 28

• When an antibody encounters a complementary antigen, the antigen binding site binds to the antigen and an antigen-antibody complex is formed.
• Since antibodies have two antigen binding sites, they can bind to two antigens, causing the pathogens to clump together, making it harder for pathogens to move throughout the body and making it easier for phagocytes to locate them, and destroy a large number of pathogens in one go.
• This is called agglutination - antibodies that carry out agglutination are called agglutinins.

Question 29

What are the three roles of antibodies?

Answer 29

1. Agglutination
2. Antibodies act as opsonins - phagocytes have receptors that are complementary to some antibodies in our blood. Therefore, these antibodies can bring a pathogen to a phagocyte, allowing the phagocyte to destroy it.
3. Antibodies act as antitoxins to the toxins produced by pathogens.

Question 30

What is the role of memory B cells?

Answer 30

• They can survive for years, and don’t release antibodies.
• They remain in our body once all of the pathogens have been removed.
• If the same pathogen invades again the antigen will bind to the antigen binding sites, and then the same response occurs (B cells are cloned).
• However memory B cells act much more quickly, meaning there are no symptoms. Cloning the huge number of memory B cells produces many more plasma cells than in the primary response, so a greater concentration of antibodies are produced.

Question 31

What is the humoral response?

Answer 31

The whole primary and secondary response

Question 32

What is the structure of T cells?

Answer 32

• T cells have receptors that are complementary to specific antigens - but only antigens on antigen presenting cells (as when they are processed their shape changes slightly).

Examples of antigen presenting cells are:
1. A B cell after binding to a pathogens antigen becomes an APC.
2. Phagocytes can become APCs after phagocytosis, as after hydrolysis of the pathogen, some phagocytes present antigens on their surface.
3. Body cells when infected with viruses, produce antigens and present them.

Question 33

Role of T cells in the immune response:

Answer 33

• A naive helper T cell can bind to an antigen presenting phagocyte, forming cloned helper T cells.
  1. Cloned helper T cells can bind to antigens on the surface of a B cell, stimulating the B cell to clone.
  2. Cloned helper T cells can release chemicals that attract phagocytes to a pathogen (stimulating phagocytosis).
  3. Cloned helper T cells can stimulate killer T cells to destroy infected body cells.
  4. Cloned helper T cells can develop into memory T cells, which is they encounter the antigen a second time, divide to form a huge number of killer T cells, resulting in a much quicker immune response.

Question 34

How do cloned helper T cells stimulate killer T cells to destroy infected body cells?

Answer 34

• Killer T cells have complementary receptors to antigens presented on infected (by a virus) body cells and bind, releasing a protein called perforin.
• Perforin causes holes to form in the infected cells cell surface membrane, increasing it’s permeability and killing the cell.

Question 35

What is the cell-mediated response?

Answer 35

The immune response involving T cells

Question 36

What is the role of regulatory T cells?

Answer 36

They identify when all foreign antigens have been eliminated, then shut down the immune response to keep our own body cells safe.

Question 37

What is the specific immune response?

Answer 37

The humoral response and the cell-mediated response (B and T cells).

Question 38

What is passive immunity?

Answer 38

When antibodies are introduced in the body:
- it is fast acting, but doesn’t result in any memory cells or new antibodies
- antibodies aren’t replaced when broken down so there is no long term immunity

• Natural passive immunity: antibodies in the placenta and breast milk
• Artificial passive immunity: antibody injections

Question 39

What is active immunity?

Answer 39

When the body produces it’s own antibodies in response to exposure to an antigen as part of the specific immune response:
- slow acting, but results in memory cells that provide long term immunity

• Natural active immunity: exposure to pathogens
• Artificial active immunity: vaccines (which contain a small amount of antigens). Vaccination programs provide herd immunity. However, it is difficult to eradicate diseases as the pathogen mutates, changing the antigens, so the vaccine contains the wrong antigens and antibodies produced are not complementary.