Topic 5 - Cell recognition and the Immune System

Question 1

What are the two types of defence mechanisms?

Answer 1

Non-specific
Specific

Question 2

What is the non-specific defence mechanism?
Give two examples.

Answer 2

The response is immediate and the same for all pathogens.
Physical barriers (skin)
Phagocytosis

Question 3

What is the specific defence mechanism?
Give two examples.

Answer 3

The response is slower and specific to each pathogen.
Cell mediated response (T lymphocytes).
Humoral response (B lymphocytes)

Question 4

Describe how the skin acts a physical barrier against pathogens?

Answer 4

Outer layer consists of flat, dead cells that consist mainly of the protein keratine. Microorganisms cannot penetrate this tough barrier.

Question 5

Describe how the nose and gas exchange system acts a physical barrier against pathogens?

Answer 5

Lined with cilia, which are bathed in mucus. The mucus will trap pathogens and the cilia will waft the mucus up to the throat where it is then swallowed into the stomach.

Question 6

Describe how the stomach acts as a physical barrier against pathogens?

Answer 6

Produces stomach acid which kills pathogens

Question 7

Describe the stages of phagocytosis.

Answer 7

1. Chemical products of the pathogen act as attractants, causing the phagocyte to move towards the pathogen.
2. Phagocytes attach themselves to the surface of the pathogen.
3. The phagocyte engulfs the pathogen to form a vesicle called a phagosome.
4. Lysosomes, which contain hydrolytic enzymes, move towards the phagosome and fuse with it.
5. The lysosome releases the hydrolytic enzymes into the phagosome. The enzymes hydrolyse the molecules that make up the pathogen into smaller soluble products.
6. The soluble products from the hydrolysis of the pathogen are then ‘presented’ on the cell membrane of the phagocyte.
7. Inflammation occurs at the site of infection due to the release of histamine. Th swollen area contains dead pathogens and phagocytes that have died after ingesting many pathogens.

Question 8

Describe the process of the humoral response.

Answer 8

1. Pathogens enter and circulate in the bloodstream. Phagocytosis occurs.
2. Pathogen antigens circulate in bloodstream.
3. Specific B lymphocytes engulf pathogenic antigens.
4. Foreign antigens are displayed on the surface of B lymphocytes.
5. Activated T helper cells bind to the presented / processed foreign antigens.
6. Activated T helpers activate one B lymphocyte to divide by mitosis.
7. The clones differentiate to form plasma B and memory B lymphocytes.
8. The plasma B cells produce thousands of specific antibodies every second.
9. The antibodies are released into the bloodstream and bind with antigens on the invading pathogen, forming antigen-antibody complexes. This causes the destruction of the pathogen (primary response).
10. The memory B cells remain and are used for subsequent invasions by the same pathogen (secondary response).

Question 9

Whats the difference in the quantity and speed of the primary and secondary response?

Answer 9

Antibody’s are produced slower and produce less antibody’s in the primary response.

Question 10

Why is the secondary response faster?

Answer 10

Memory cells are already present so can turn into plasma cells.

Question 11

What biological molecule are antibodies?

Answer 11

Proteins

Question 12

Describe the structure of an antibody.

Answer 12

‘Y’ shaped protein made from 4 polypeptide chains two of which are heavy (big), the other two are light (small).
They have a variable region which differs between each type of antibody and a constant region which is the same in all antibodies.
The polypeptide chains are held together by disulphide bridges.

Question 13

What is agglutination?

Answer 13

The clumping of pathogens together.

Question 14

What is the difference between polyclonal and monoclonal antibodies?

Answer 14

Monoclonal - isolating and cloning a single type of antibody from a single type of plasma clone.
Polyclonal - many different types of B cells activated and many different types of antibodies cloned.

Question 14

Describe the process of the direct ELISA test.

Answer 14

1. Monoclonal antibodies bind to bottom of test plate.
2. Antigen molecules in sample bind to antibody. Rinse excess.
3. Mobile antibody with ‘reporter enzyme’ attached binds to antigens that are ‘fixed’ on the monoclonal antibodies. Rinse excess.
4. Add substrate complementary to ‘reporter enzyme’.
5. Colour change if positive result.

Question 15

Describe the process of the indirect ELISA test.

Answer 15

1. Antigens bind to bottom of test plate.
2. Antibodies in sample bind to antigen. Wash away excess.
3. Secondary antibody with ‘reporter enzyme’ attached binds to primary antibodies from the sample.
4. Add substrate for reporter enzyme.
5. Colour change for positive result.

Question 16

What is the principle of the direct ELISA test?

Answer 16

To detect the presence of a specific antigen.

Question 17

What is the principle of the indirect ELISA test?

Answer 17

To detect the presence of a specific antibody against a specific antigen.

Question 17

What is an ethical issue with the use of monoclonal antibodies?

Answer 17

Production involves animals.

Question 18

Describe how monoclonal antibodies are formed.

Answer 18

1. Mouse infected with chosen antigen to stimulate B lymphocytes (plasma cells) to make a particular antibody.
2. B lymphocytes produce antibodies.
3. Myelomas ( malignant white blood cell tumor cells in the bone marrow) are able to divide.
4. B lymphocytes are removed from the mouse and combined with the myelomas to make a hybridoma cell.
5. Hybridoma cells can divide and produce the desired antibody.
6. Hybridoma cells are cloned in a culture medium. Any unbound myeloma cells die.
7. Large amounts of the monoclonal antibodies are collected and purified by centrifugation, filtration and chromatography.

Question 19

What is the principle of a vaccine?

Answer 19

A preparation / injection of antigens or attenuated microorganisms that have been treated to make them harmless.
This stimulates the formation of memory cells and when carried out on large scales provides ‘herd immunity’.

Question 20

Why are vaccines specific?

Answer 20

Specific to one antigen
Stimulates one type of B cell
Make complementary antibodies / memory cells.