cell recognition and immune system

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 and 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. lysosomes digest pathogen
5. phagocyte adsorbs 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)

enhances recognition by TH 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 (inflammation, phagocytosis) = same for all pathogens ; immediate

specific (B and T lymphocytes) = complementary pathogen ; time lag

Question 5

2 types of specific immune response

Answer 5

• cell mediated
• humoral

Question 6

outline process of cell-mediated response

Answer 6

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 7

outline process of humoral response

Answer 7

1. complementary TH lymphocytes bind to foreign antigen or 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 bridge, 2 longer ‘heavier chains’
• binding sites on variable region of light chains have specific tertiary structure complementary to an antigen
• rest of the molecule known as constant region

Question 9

how do antibodies lead to the destruction of pathogen

Answer 9

formation of antigen-antibody complex results in AGGLUTINATION which enhances phagocytosis

Question 10

what are monoclonal antibodies

Answer 10

antibodies produced from a single clone of B cells

Question 11

what are memory cells

Answer 11

• specialised TH / 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 12

secondary immune response compared to primary immune response

Answer 12

secondary
- has 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 and disulphide 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

examples of passive natural

Answer 15

antibodies in breast milk/across placenta

Question 16

examples of passive artificial

Answer 16

antivenom, needle stick injections

Question 17

examples of active natural

Answer 17

humoral response to infection

Question 18

examples of active artificial

Answer 18

vaccination

Question 19

passive immunity

Answer 19

• no memory cells and the antibodies are not replaced after breaking down = short-term
• immediate response
• antibodies from external source
• direct contact with antigen not necessary

Question 20

active immunity

Answer 20

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

Question 21

explain principle of vaccination

Answer 21

1. vaccine contains dead/inactive form of 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 destroyed before causing symptoms

Question 22

what is herd immunity

Answer 22

vaccinating large proportion of population reduces available carriers of the pathogen
protects individuals who have not been vaccinated e.g. those with weak immune system

Question 23

suggest ethical issues surrounding use of vaccines

Answer 23

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

Question 24

describe the structure of HIV

Answer 24

• genetic material (2xRNA) and viral enzymes (integrase and reverse transcriptase) surrounded by capsid
• surrounded by viral envelope derived from host cell membrane
• GP120 attachment protein on surface

Question 25

how does HIV result in the symptoms of AIDS

Answer 25

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 and suffer from secondary diseases/infections

Question 26

why are antibiotics ineffective against viruses

Answer 26

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 27

suggest the clinical application of monoclonal antibodies

Answer 27

• 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 28

explain the principle of a direct ELISA test

Answer 28

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 29

explain the principle of an indirect ELISA test

Answer 29

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 sample
4. add substrate for reporter enzyme. positive result: colour change

Question 30

suggest ethical issues around monoclonal antibodies

Answer 30

• production involves animals
• drug trials against arthritis and leukaemia resulted in multiple organ failure