2.4-Cell recognition and the immune system

Question 1

What is an antigen?

Answer 1

• Cell-surface molecule which stimulates the immune response.
• Usually (glyco)protein, sometimes (glyco)lipid or polysaccharide.
• Immune system recognises as ‘self’ or ‘non-self’ = enables indentification of cells from other organisms of sam speciec, pathogen, 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

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

Answer 3

• Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis).
• Enhances recognition bu T(H) 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:
- immediate
- (inflammation, phagocytosis) = same for all pathogens
Specific:
- time lag
- (B & T lymphocytes) = complementary pathogen

Question 5

Name the 2 types of specific immune response.

Answer 5

• Cell-mediated
• Humorol

Question 6

Ouline the process of the cell-mediated response.

Answer 6

1. Complementary T(H) lymphocytes bind to foreign antigen on APC.
2. Release cytokines that stimulate:
   a. clonal expansion of complementary T(H) 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 7

Outline the process of the humoral response.

Answer 7

1. Complementary T(H) lymphocytes bind to foeign 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 8

What is an antibody?

Answer 8

• 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 9

How do antibodies lead to destruction of a pathogen?

Answer 9

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

Question 10

What are monoclonal antibodies?

Answer 10

Antibodies producd from a single clone of B cells.

Question 11

What are memory cells?

Answer 11

• Specialised T(H)/ 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.

Question 12

Contrast the primary and secondary immune response.

Answer 12

Secondary response:
- 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 13

What causes antigen variability?

Answer 13

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 14

Explain how antigen variability affects the incidence of disease.

Answer 14

• 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 15

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

Answer 15

• Both involve antibodies
• Can both be natural or artificial
  Passive natural: antibodies in breast milk/ across placenta
  Passive artificial: anti-venom, needle sick injections
  Active natural: humoral response to infection
  Active artificial:vaccination

Question 16

Contrast passive and active immunity.

Answer 16

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 atigen necessary

Question 17

Explain the principles of vaccination.

Answer 17

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

Question 18

What is herd immunity?

Answer 18

• 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 19

Suggest some ethical issues surrounding the use of vaccines.

Answer 19

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

Question 20

Describe the structure of HIV.

Answer 20

• 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.