Cell recognition and the immune system - Cells

Question 1

What is an antigen?

Answer 1

● Cell-surface molecule which stimulate 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 same species, pathogens, toxins & abnormal body cells.

Question 2

How does phagocytosis destroy pathogens?

Answer 2

1. Phagocyte 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

Give 2 differences between specific and nonspecific immune responses.

Answer 3

nonspecific (inflammation, phagocytosis) = same for all pathogens

specific (B & T lymphocytes) = complementary pathogen

nonspecific = immediate specific = time lag

Question 3

Explain the role of antigen-presenting cells (APCs).

Answer 3

Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis).

Enhances recognition by TH cells, which cannot directly interface with pathogens/ antigens in body fluid.

Question 4

Name the 2 types of specific immune response.

Answer 4

● cell-mediated ● humoral

Question 5

Outline the process of the cell-mediated response.

Answer 5

1. Complementary TH lymphocytes bind to foreign antigen on APC.
2. Release cytokines that stimulate:
   a) clonal expansion of complementary TH cells (rapid
   mitosis): become memory cells or trigger humoral
   response.
   b) clonal expansion of cytotoxic T cells (TC): secrete
   enzyme perforin to destroy infected cells.

Question 6

Outline the process of the humoral response.

Answer 6

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

Question 7

What is an antibody?

Answer 7

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 the constant region.

Question 8

How do antibodies lead to the destruction of a pathogen?

Answer 8

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

Question 9

What are monoclonal antibodies?

Answer 9

Antibodies produced from a single clone of B cells.

Question 10

What are memory cells?

Answer 10

● Specialised TH/ 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 11

Contrast the primary and secondary immune response.

Answer 11

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

Question 12

What causes antigen variability?

Answer 12

1. Random genetic mutation changes DNA base sequence.
2. Results in different sequence of codons on mRNA
3. Different primary structure of antigen = H-bonds,
   ionic bonds & disulfide bridges form in different
   places in tertiary structure.
4. Different shape of antigen.

Question 13

Explain how antigen variability affects the incidence of disease.

Answer 13

● 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 14

Compare passive and active immunity. Give examples of both types.

Answer 14

● both involve antibodies
● can both be natural or artificial

passive natural: antibodies in breast milk/ across placenta passive artificial: anti-venom, needle stick injections active natural: humoral response to infection
active artificial: vaccination

Question 15

Contrast passive and active immunity.

Answer 15

Passive:
no memory cells & antibodies not replaced when broken down = short-term
immediate
antibodies from external source
direct contact with antigen not necessary

Active:
memory cells produced = long-term
time lag
lymphocytes produce antibodies
direct contact with antigen necessary

Question 16

Explain the principles of vaccination.

Answer 16

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 &
   produces higher concentration of antibodies.
4. Pathogen is destroyed before it causes symptoms.

Question 17

What is herd immunity?

Answer 17

Vaccinating 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 18

Suggest some ethical issues surrounding the use of vaccines.

Answer 18

● production may involve use of animals ● potentially dangerous side-effects
● clinical tests may be fatal
● compulsory vs opt-out

Question 19

Describe the structure of HIV.

Answer 19

● Genetic material (2 x RNA) & viral enzymes (integrase & reverse transcriptase) surrounded by capsid.
● Surrounded by viral envelope derived from host cell membrane.
● GP120 attachment proteins on surface.

Question 20

How does HIV result in the symptoms of AIDS?

Answer 20

1. Attachment proteins bind to complementary CD4 receptor 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 21

Why are antibiotics ineffective against viruses?

Answer 21

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 22

Suggest the clinical applications of monoclonal antibodies.

Answer 22

● 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 e.g. cancer cells due to specificity of tertiary structure of binding site.

Question 23

Explain the principle of a direct ELISA test.

Answer 23

detects presence of a specific antigen
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 for reporter enzyme. Positive result: colour
change.

Question 24

Explain the principle of an indirect ELISA test.

Answer 24

detects presence of an antibody against a specific antigen

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. Positive result: colour
   change.

Question 25

Suggest some ethical issues surrounding the use of monoclonal antibodies.

Answer 25

● Production involves animals. ● Drug trials against arthritis &
leukaemia resulted in multiple organ failure.

Question 26

How is RNA turned into DNA

Answer 26

Reverse transcriptase