2.4 Cell Recognition & the Immune System

Question 1

What is an antigen?

Answer 1

• Cell-surface molecule which stimulates an immune response.
• Usually (glyco) protein, sometimes (glyco)lipid or polysaccharide.
• Immune system recognises as “self” or “non-self” = enables identification of cells from other organisms of the same species, pathogens, toxins & abnormal body cells.

Question 2

How does phagocytosis destroy pathogens?

Answer 2

1. Phagocytye moves towards pathogen via chemotaxis.
2. Phagocyte engulfs pathogen via endocytosis to form a phagosome.
3. Phagosome fuses with lysosome (phagolysosome).
4. Lysozymes digest pathogen.
5. Phagocyte absorbs the products from pathogen hydrolysis.

Question 3

Role of antigen-presenting cells (APCs).

Answer 3

• Macrophage displays antigen from the pathogen on its surface after hydrolysis in phagocytosis.
• Enhances recognition by T helper cells, which cannot directly interface with pathogens/antigen body fluid.

Question 4

Details about the specific immune response.

Answer 4

Involves B & T lymphocytes that are complementary to specific pathogens, and has a time lag.

Question 5

Details about the nonspecific immune response.

Answer 5

Includes inflammation and phagocytosis, same for all pathogens, immediate.

Question 6

What do T helper lymphocytes bind to during the cell-mediated response?

Answer 6

Foreign antigen on antigen-presenting cells (APC).

Question 7

What do T helper lymphocytes release to stimulate other immune responses?

Answer 7

Cytokines.

Question 8

What happens to complementary T helper cells after clonal expansion?

Answer 8

They become memory cells or trigger the humoral response.

Question 9

What do cytotoxic T cells secrete to destroy infected cells?

Answer 9

The enzyme perforin.

Question 10

What do T helper lymphocytes bind to in the humoral response?

Answer 10

Foreign antigen on antigen-presenting T cells.

Question 11

What do T helper lymphocytes release to stimulate B lymphocytes?

Answer 11

Cytokines.

Question 12

What process do B lymphocytes undergo after stimulation in the humoral response?

Answer 12

Clonal expansion.

Question 13

What do B cells differentiate into during the humoral response?

Answer 13

Plasma cells.

Question 14

What do plasma cells secrete in the humoral response?

Answer 14

Antibodies with a complementary variable region to the antigen.

Question 15

What is an antibody?

Answer 15

Proteins secreted by plasma cells with specific binding sites (on variable region of light chains have specific tertiary structures) complementary to an antigen.

Question 16

Describe the structure of an antibody.

Answer 16

Quaternary structure: 2 ‘light chains’ held together by disulphide bridges, 2 longer ‘heavy chains’.
Rest of the molecule is known as the constant region.

Question 17

How do antibodies lead to the destruction of a pathogen?

Answer 17

Formation of antigen-antibody complex results in agglutination, which enhances phagocytosis.

Question 18

What are monoclonal antibodies?

Answer 18

Antibodies produced from a single clone of B cells.

Question 19

What are memory cells?

Answer 19

Specialised TH/B cells produced from primary immune response that can divide rapidly upon re-exposure.

Question 20

What is the primary immune response?

Answer 20

Initial response to a pathogen that results in slower antibody production.

Question 21

What is the secondary immune response?

Answer 21

Faster and more effective response to a previously encountered pathogen.

Question 22

What causes antigen variability?

Answer 22

1. Random genetic mutation changes DNA base sequence.
2. Results in different sequences of codons on mRNA.
3. Different primary structure of antigen.
4. Different shape of antigen.

Question 23

Explain how antigen variability affects the incidence of diesease.

Answer 23

• Memory cells no longer complementary to antigen = individual not immune = can catch disease more than once.
• Many varieties of a pathogen = difficult to develop vaccine containing all antigen types.

Question 24

What is passive immunity?

Answer 24

Involves antibodies from an external source, can be natural (e.g., breast milk) or artificial (e.g., anti-venom).

Question 25

What is active immunity?

Answer 25

Involves the body's own lymphocytes producing antibodies, can be natural (e.g., infection) or artificial (e.g., vaccination).

Question 26

Explain the principles of vaccination. (4)

Answer 26

1. Vaccine contains dead/inactive form of a pathogen/antigen. 2. Triggers primary immune response. 3. Memory cells are produced and remain in bloodstream, so secondary response is rapid and produces higher concentration of antibodies. 4. Pathogen is destroyed before it causes symptoms.

Question 27

What is herd immunity?

Answer 27

Vaccinating a large proportion of the population to reduce the spread of a pathogen.

Question 28

What does herd immunity protect?

Answer 28

Individuals who have not been vaccinated.

Question 29

Ethical issues of vaccines.

Answer 29

Concerns include animal use, side effects, clinical test risks, and compulsory vaccination debates.

Question 30

Structure of HIV.

Answer 30

- Contains genetic material (2 x RNA) and viral enzymes surrounded by a capsid. - Surrounded by a viral envelope derived from the host cell membrane. - GP120 attachment proteins on the surface.

Question 31

How does HIV result in symptoms of AIDS?

Answer 31

1. Attachment proteins bind to complementary CD4 receptors on TH cells. 2. HIV particles replicate inside TH cells, killing or damaging them. 3. AIDS develops when there are too few TH cells for the immune system to function. 4. Individuals cannot destroy other pathogens & suffer from secondary diseases/ infections.

Question 32

Why are antibiotics ineffective against viruses?

Answer 32

- Antibiotics often work by damaging the murein cell walls to cause osmotic lysis. Viruses have no cell wall. - Viruses replicate inside host cells = difficult to destroy them without damaging normal body cells.

Question 33

Clinical applications of monoclonal antibodies.

Answer 33

Used in pregnancy tests, diagnostic procedures, and targeted treatments for diseases like cancer.

Question 34

Explain the principle of a direct ELISA test.

Answer 34

Detects presence of a specific antigen. 1. Monoclonal antibodies bind to bottom of a test plate. 2. Antigen molecules in sample bind to antibody. Rinse excess. 3. Mobile antibody with a 'reporter enzyme' attached, binds to antigens that are fixed on the monoclonal antibodies. Rinse excess. 4. Add substrate for the reporter enzyme. Positive result: colour change.

Question 35

What does the Indirect ELISA test detect?

Answer 35

Presence of an antibody against a specific antigen.

Question 36

What is the first step in the Indirect ELISA test?

Answer 36

Antigens bind to the bottom of the test plate.

Question 37

What happens after antibodies in the sample bind to the antigen in the Indirect ELISA test?

Answer 37

Excess antibodies are washed away.

Question 38

What type of antibody is used after the primary antibodies in the Indirect ELISA test?

Answer 38

A secondary antibody with a 'reporter enzyme' attached.

Question 39

What is added to the Indirect ELISA test to indicate a positive result?

Answer 39

Substrate for the reporter enzyme.

Question 40

What indicates a positive result in the Indirect ELISA test?

Answer 40

A colour change.

Question 41

Ethical issues of monoclonal antibodies.

Answer 41

Concerns include animal involvement and risks associated with drug trials.