6. Immunity

Question 1

What are the main types of defence mechanisms?

Answer 1

(a) Specific - Can distinguish between pathogens and produce long-term immunity. Involved two types of white blood cells - T and B lymphocytes
(b) Non-specific - Respond in the same way to every pathogen. Phagocytosis and natural barriers (skin, stomach acid, mucus, platelets)

Question 2

How does your body prevent itself from destroying your own cells?

Answer 2

• There are millions of lymphocytes each able to recognise different pathogens
• These lymphocytes are constantly colliding with other cells in a fetus
• Infection in a fetus is rare because it is protected from the outside world by the mother and the placenta
• Lymphocytes therefore collide exclusively with the body’s own cells
• If the lymphocytes have receptors that exactly fit the body’s own cells, they will be destroyed or suppresed
• The remaining lymphocytes are those that fit non-self material

Question 3

What are the stages of phagocytosis?

Answer 3

1. The phagocyte is attracted to the pathogen by chemoattractants and moves towards it along a concentration gradient
2. The phagocyte binds to the surface of the pathogen, forming a phagosome
3. The lysosomes migrate towards the phagosome and fuse with it
4. They release their lytic enzymes into the phagosymes so the bacterium is broken down
5. The breakdown products are absorbed into the cytoplasm of the phagocyte. Any useful products are released into the body

Question 4

How do phagocytes travel around the body?

Answer 4

Most travel in the blood but can move out of the blood vessels into other tissues

Question 5

Why does phagocytosis cause inflammation at the site of infection?

Answer 5

The swollen area contains dead pathogens and phagocytes, known as pus. Inflammation occurs because histamine is released which causes dilation of the blood vessels, which speeds up the delivery of phagocytes to the site of infection

Question 6

What is an antigen?

Answer 6

Non-self material, usually in the form of a protein, that is capable of triggering an immune response

Question 7

Where do T cells mature?

Answer 7

In the thymus gland, but they are formed from stem cells found in the bone marrow

Question 8

What type of immunity are T cells associated with?

Answer 8

Cell-mediated immunity - they respond to an organism’s cells that have been invaded by non-self material

Question 9

How can T lymphocytes distinguish between normal cells and invaded cells?

Answer 9

• Phagocytes present the antigens from the pathogens they digest on their own cell-surface membranes
• Body cells invaded by a virus present some of the viral antigens on their own cell-surface membranes
• Cancer cells present antigens on their cell-surface membranes

These are all called antigen-presenting cells because they can all present antigens on their cell-surface membranes

Question 10

How do T lymphocytes respond when a pathogen invades the body?

Answer 10

1. Pathogens invade body cells and are taken in by phagocytes
2. The phagocyte places antigens from the pathogen on its cell-surface membrane
3. Receptors on the T helper cells fit onto these antigens
4. This activates T cells to divide rapidly by mitosis. The clones cells differentiate to:
   a) form memory cells
   b) stimulate phagocytes to engulf pathogens
   c) stimulate B cells to divide
   d) kill infected cells

Question 11

How do T cells kill infected cells?

Answer 11

• They produce a protein that makes holes in the cell-surface membrane
• The cell is then freely permeable to all substances so they die
• This is very effective against viruses, which need living cells in which to multiply and survive

Question 12

What does humoral immunity involve?

Answer 12

Antibodies dissolved in the blood and tissues fluid. The antibodies are produced by B lymphocytes

Question 13

How do B cells respond when the body is invaded by a pathogen?

Answer 13

1. The surface antigens of the invading pathogens are taken up by the B cells
2. The B cells process the antigens and present them on their surface
3. T helper cells attach themselves to the processed antigens on the B cells therefore activating them
4. The B cells divide rapidly by mitosis to produce memry cells or plasma cells
5. The plasma cells produce antibodies that fit exactly to the antigens on the pathogen’s surface and destroy them (primary immune response)
6. The memory cells respond to future infections by dividing rapidly and developing into plasma cells that produce antibodies (secondary immune response)

Question 14

How does antigenic variability affect the secondary immune response?

Answer 14

The antibodies made will not be complementary to the antigens present so the pathogen will not be destroyed. This will result in a much slower immune response as new antibodies are made.

Question 15

What are antibodies?

Answer 15

Proteins (consisting of four polypeptide chains) synthesized by B cells

Question 16

Why is there a huge variety of antibodies?

Answer 16

Because they are made of proteins - molecules that occur in an almost infinite number of forms

Question 17

How is an antibody structured?

Answer 17

Heavy and light chains (light chains make up the variable region)
Constant region (same in all antibodies)
Variable region (different on different antibodies due to a different sequence of amino acids that forms a very specific 3D shape)
Antigen binding sites (both sides of the variable region.

Question 18

What is a monoclonal antibody?

Answer 18

An antibody synthesized by only one type of B cell and complementary to a single antigen

Question 19

How can monoclonal antibodies be used?

Answer 19

• Separation of a chemical from a mixture
• Immunoassay (calculating the amount of a substance in a mixture) e.g. preganc testing, AIDS
• Cancer treatment (antibodies only attach themselves to cancer cells so can be used to activate a cytotoxic drug without damaging other cells)
• Transplant surgery (monoclonal antibodies supress T cells to prevent rejection)

Question 20

How are monoclonal antibodies produced?

Answer 20

1. A mouse in injected with antigens
2. This produces an immune response, and the B cells produce plasma cells which produce antibodies
3. The antibodies are extracted fused with tumour cells to enable them to divide rapidly outside of the body
4. Detergent is added to break down cell-surface membranes to enable them to fuse together more easily
5. The fused cells are separated and each single cell is cultured to form a clone
6. The clones are observed under a microscope
7. If they are producing the desired monoclonal antibodies, they are grown on a large scale and the antibodies are extracted from the growth medium
8. The antibodies must undergo a process called humanisation because they came from mouse tissue

Question 21

Read over the ethical use of monoclonal antibodies.

Answer 21

Done!

Question 22

What is passive immunity?

Answer 22

The introduction of antibodies into individuals from an outside source. Once broken down, the antibodies can not be reproduced so this type of immunity is generally short-lived.

Question 23

What is active immunity?

Answer 23

The stimulation of the production of antibodies by the individuals’ own immune system. This generally produces long-term immunity

Question 24

What is vaccination?

Answer 24

The introduction of a substance into the body with the intention of stimulating active immunity against a particular disease

Question 25

What are the features of a successful vaccination programme?

Answer 25

• Economically available vaccine in sufficient quantities to immunise all of the vulnerable population
• Few side-effects which might discourage individuals from being vaccinated
• Means of producing, storing and transporting the vaccine
• Means of administering the vaccine with enough people equipped with appropriate training
• It must be possible to immunise the vast majority of the population at the same time to produce herd immunity

Question 26

Why does vaccination not eliminate disease?

Answer 26

• Fails to produce immunity in certain individuals e.g. if they have a defective immune system
• Individuals may develop the disease immediately following vaccination before their immunity levels are high enough to prevent it, and may then harbour the pathogen and reinfect others
• The pathogen may mutate frequently and have great antigenic variability
• There may be too may varieties of a particular pathogen to produce a vaccine
• Certain pathogens ‘hide’ from the body’s immune system e.g. by hiding out of reach like in the intestines (cholera)
• Individuals may have objections to vaccinations

Question 27

Why is it difficult to control cholera by means of vaccination?

Answer 27

• The intestines are not easily reached by the immune system and any oral treatment e.g. antibiotics is flushed from the intestines by diarrhoea
• Antigenic variability makes it difficult to develop a long-lasting vaccine
• Mobile populations make it difficult to insure that all individuals are vaccinated

Question 28

Why is it difficult to control tuberculosis by means of vaccination?

Answer 28

• A increase in HIV means more people have impaired immune systems so they are more likely to catch TB
• Mobile populations
• Crowded cities
• Proportion of elderly people in the population is increasing, in which vaccination is often less effective at stimulating immunity

Question 29

Read Application and HSW information pages 113-115.

Answer 29

Done!