2.4 Cell Recognition And The Immune System

Question 1

What is an antigen?

Answer 1

• Cell surface membrane which stimulate immune response
• Usually glycoprotein, sometimes glycolipid or polysaccharide
• Immune system recognises as ‘self’ or ‘non-self’ = enables identification of cells from other organisms of same species, pathogens, toxins and abnormal body cells

Question 2

How does phagocytosis destroy pathogens?

Answer 2

1. Phagocyte moves toward pathogen via chemotaxis
2. Phagocyte engulfs pathogen via endocytosis
3. Phagosome fuses with lysosome
4. Lysozymes digest pathogens
5. Phagocyte absorbs the products from pathogen hydrolysis

Question 3

Explain the role of antigen-presenting cells

Answer 3

• Macrophage displays antigen from pathogen on its surface
• Enhances recognition of helper T cells, which cannot directly interface with pathogens/antigens in body fluid

Question 4

Give 2 differences between specific and nonspecific immune responses

Answer 4

• Nonspecific - same for all pathogens, specific - complementary pathogen
• Nonspecific - immediate, specific - time lag

Question 5

Name two types of specific immune response

Answer 5

• Cell mediated
• Humoral

Question 6

Outline the process of the cell-mediated response

Answer 6

1. Complementary helper T lymphocytes bind to foreign antigen on APC
2. Release cytokines that stimulate clonal expansion of complementary helper T cells and clonal expansion of cytotoxic T cells

Question 7

What is clonal expansion of complementary helper T cells?

Answer 7

Become memory cells or trigger humoral response

Question 8

What is clonal expansion of cytotoxic T cells?

Answer 8

Secrete the enzyme perforin to destroy infected cells

Question 9

Outline the process of the humoral response

Answer 9

1. Complementary T helper lymphocytes bind to foreign antigen on antigen-presenting T cells
2. Release cytokines that stimulate clonal expansion of complementary B lymphocytes
3. B cells differentiate into plasma cells
4. Plasma cells secrete antibodies with complementary variable region to antigen

Question 10

What is an antibody?

Answer 10

• Proteins secreted by plasma cells
• Quaternary structure: 2 ‘light chains’ held together by disulfide bridges, 2 longer ‘heavy chains’
• Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen
• The rest of the molecule is known as a constant region

Question 11

How do antigens lead to the destruction of a pathogen?

Answer 11

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

Question 12

What are monoclonal antibodies?

Answer 12

Antibodies produced from a single clone of B cells

Question 13

What are memory cells?

Answer 13

• Specialised T helper cells/ B cells produced from primary immune response
• Remain in low levels in the blood
• Can divide very rapidly by mitosis if organism encounters the same pathogen again

Question 14

Contrast the primary and secondary immune response

Answer 14

• Faster rate of antibody production
• Shorter time lag between exposure and antibody production
• Higher concentration of antibodies
• Antibody level remains higher after the secondary response
• Pathogen usually destroyed before any symptoms

Question 15

What causes antigen variability?

Answer 15

• Random genetic mutation changes in DNA base sequence
• Results in different sequence of codons on mRNA
• Different primary structure of antigen = H bonds, ionic bonds and disulfide bridges form in different places in the tertiary structure
• Different shape of antigen

Question 16

Explain how antigen variability affects the incidence of disease

Answer 16

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

Question 17

Compare passive and active immunity

Answer 17

• Both involve antibodies
• Can be both natural or artificial

Question 18

Examples of passive natural immunity

Answer 18

Antibodies in breast milk/across placenta

Question 19

Examples of passive artificial immunity

Answer 19

Anti-venom, needle stick injections

Question 20

Examples of active natural immunity

Answer 20

Humoral response to infection

Question 21

Examples of active artificial immunity

Answer 21

Vaccination

Question 22

Contrast passive and active immunity

Answer 22

• Passive: no memory cells and antibodies not replaced when broken down = short term, active: memory cells produced = long term
• Passive: immediate, active: time lag
• Passive: antibodies from external source, active: lymphocyte produce antibodies
• Passive: direct contact with antigen not necessary, active: direct contact with antigen necessary

Question 23

Explain the principles of vaccination

Answer 23

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

Question 24

What is herd immunity?

Answer 24

• Vaccinating a large proportion of population reduces available carriers of the pathogen
• Protects individuals who have not been vaccinated e.g. those with a weak immune system

Question 25

Suggest some ethical issues surrounding the use of vaccine

Answer 25

- Production may involve use of animals - Potentially dangerous side effects - Clinical tests may be fatal - Compulsory vs opt out

Question 26

Describe the structure of HIV

Answer 26

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

Question 27

How does HIV result in symptoms of AIDs?

Answer 27

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

Question 28

Why are antibiotics ineffective against viruses?

Answer 28

- Antibiotics often work by damaging 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 29

Suggest the clinical applications of monoclonal antibodies

Answer 29

- Pregnancy tests by detecting HCG hormones in urine - Diagnostic procedures e.g. ELISA test - Targeted treatment by attaching drug to antibody so that it only binds to cells with abnormal antigen

Question 30

Explain the principle of the direct ELISA test

Answer 30

1. Monoclonal antibodies bind to the bottom of the test plate 2. Antigen molecules in the sample bind to the 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 for reporter enzyme. Positive result: colour change

Question 31

What does a direct ELISA test do?

Answer 31

Detect the presence of a specific antigen

Question 32

Explain the principle of an indirect ELISA test

Answer 32

1. Antigens bind to the bottom of the 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. Positive result: colour change

Question 33

What does an indirect ELISA test do?

Answer 33

Detects the presence of an antibody against a specific antigen

Question 34

Suggest some ethical issues surrounding the use of monoclonal antibodies

Answer 34

- Production involves animales - Drug trials against arthritis and leukaemia resulted in multiple organ failures