2.4 Cell Recognition & the Immune System

Question 1

What is an antigen?

Answer 1

Cell surface molecule which stimulate immune response

Question 2

How does the immune system recognise antigens?

Answer 2

As “self” or “non-self”

Question 3

What is antigen recognition useful for?

Answer 3

Enables identification of cells from other organisms, pathogens, toxins and abnormal body cells

Question 4

Describe how phagocytosis destroy pathogens.

Answer 4

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

Question 5

What does APCs stand for?

Answer 5

Antigen-Presenting Cells

Question 6

What is an antigen-presenting cell?

Answer 6

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

Question 7

Why are APCs necessary?

Answer 7

Enhances recognition by helper T cells which cannot directly interface with pathogens/antigens in bidy fluid

Question 8

Give two differences between specific and nonspecific immune responses.

Answer 8

Nonspecific is immediate - specific has a time lag
Nonspecific is the same for all pathogens - specific is complementary to pathogens

Question 9

Name the two types of specific immune response.

Answer 9

Cell mediated
Humoral

Question 10

Describe the process of cell-mediated response.

Answer 10

1. Complementary helper T lymphocytes bind to foreign antigen on APC
2. Release cytokines that stimulate:
   a) clonal expansion of complementary helper T cells - become memory cells ir trigger humoral response
   b) clonal expansion if cytotoxic T cells - secrete enzyme perforin to destroy infected cells

Question 11

Describe the process of the humoral response.

Answer 11

1. Complementary helper T lymphocytes bind to foreign antigen in 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 12

What is an antibody?

Answer 12

Proteins secreted by plasma cells

Question 13

Describe the quarternary structure of an antibody.

Answer 13

2 “light chains” held by disulfide bridges and 2 longer “heavy chains”

Question 14

Describe the regions of an antibody.

Answer 14

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 15

How do antibodies lead to the destruction of a pathogen?

Answer 15

Formation if antigen-antibody complex results in aggultination which enhances phagocytosis

Question 16

What are monoclonal antibodies?

Answer 16

Antibodies produced from a single clone of B cells

Question 17

What are memory cells?

Answer 17

Specialised helper T cells or B cells produced from primary immune response
Remain in low levels in the blood

Question 18

What do memory cells do if organism encounters same pathogen again?

Answer 18

Divide rapidly by mitosis

Question 19

What are the advantages of the secondary immune response?

Answer 19

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

Question 20

What causes antigen variability?

Answer 20

1. Random genetic mutation changes DNA base sequence
2. Different sequence of codons on mRNA
3. Different primary structure of antigen means different disulfide bridges and H and ionic bonding in tertiary structure
4. Different shape of antigen

Question 21

Explain how antigen variability affects the incidence of disease.

Answer 21

Memory cells no longer complementary to antigen which prevents immunity so disease can be caught more than once
Many varieties of a pathogen makes a vaccine difficult to develop

Question 22

Describe similarities between passive and active immunity.

Answer 22

Both involve antibodies
Can both be natural or artificial

Question 23

Give an example of natural passive immunity.

Answer 23

Antibodies in breast milk/placenta

Question 24

Give an example of artificial passive immunity.

Answer 24

Anti-venom

Question 25

Give an example of natural active immunity.

Answer 25

Humoral response

Question 26

Give an example of artificial active immunity.

Answer 26

Vaccination

Question 27

Describe differences between passive and active immunity.

Answer 27

Active long term - passive short term Active time lag - passive immediate Active lymphocytes produce antibodies - passive antibodies from external source Active requires direct contact with antigen - passive doesnt require direct contact with antigen

Question 28

Why is active immunity long term?

Answer 28

Memory cells produced

Question 29

Why is passive immunity short term?

Answer 29

No memory cells are produced Antibodies are not replaced when broken down

Question 30

Describe how vaccinations function.

Answer 30

1. Vaccine contains dead/inactive form of a pathogen or antigen 2. Triggers primary immune response 3. Memory cells produced and remain in bloodstream 4. Pathogen is destroyed before symptoms 5. Upon reinfection, secondary response is rapid and produces higher concentration of antibodies

Question 31

What is herd immunity?

Answer 31

Vaccinating large proportion of the population reduces available carriers of the pathogen so protects those who are unvaccinated

Question 32

Suggest ethical issues surrounding vaccine use.

Answer 32

Production may involve animals Potentially dangerous side effects Clinical tests may be fatal

Question 33

Describe the structure of HIV.

Answer 33

RNA and viral enzymes surrounded by capsid Surrounded by viral envelope derived from host cell membrane GP120 attachment proteins on surface

Question 34

How does HIV result in the symptoms of AIDS?

Answer 34

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

Question 35

Why are antibiotics ineffective against viruses?

Answer 35

Antibiotics often work by damaging murein cell walls to cause osmotic lysis - viruses have no cell walls

Question 36

Why are viruses difficult to destroy?

Answer 36

Viruses replicate inside host cells so it is difficult to destroy them without damaging normal body cells

Question 37

Suggest the clinical applications of monoclonal antibodies.

Answer 37

Pregnancy tests - detection of HCG hormones in urine Diagnostic procedures e.g. ELISA test Targeted treatment by attaching drug to antibody

Question 38

Explain the principle of a direct ELISA test.

Answer 38

To detect the presence of a specific antigen

Question 39

Explain how a direct ELISA test works.

Answer 39

1. Monoclinal antibodies bind to bottom of test plate 2. Antigen molecules in sample bind to antibody and rinse excess 3. Mobile antibody with enzyme attached binds to antigens that are fixed on the monoclonal antibodies and rinse excess 4. Add substrate for enzyme - positive result is colour change

Question 40

Explain the principle of an indiret ELISA test.

Answer 40

To detect presence of an antibody against a specific antigen

Question 41

Explain how a indirect ELISA test works.

Answer 41

1. Antigens bind to bottom of test plate 2. Antibodies in sample bind to antigen and rinse excess 3. Secondary antibody with enzyme attached binds to primary antibodies from the sample 4. Add substrate for enzyme - positive result is colour change