3.2.4 Cell recognition and the immune system

Question 1

What is an antigen

Answer 1

Cell surface molecule which stimulate immune response
Usually glycoprotein or glycolipid or polysaccharide
Immune system recognises as self or non self = enabled identification of cells from other organisms of same species, pathogens, toxins and abnormal body cells

foreign protein that stimulates an immune response

Question 2

How do phagocytes destroy pathogens

Answer 2

Phagocytes in blood engulf pathogen to form a phagosome
Phagosome fuses with lysosome, lysozymes hydrolyse the pathogen
The phagocyte then presents the antigens on its surface to activate other immune system cells. The phagocyte acts as an antigen presenting cell

Question 3

Explain the role of antigen presenting cells

Answer 3

Macrophage displays an antigen from pathogen on its surface
Enhances recognition by TH cells which cannot directly interface with pathogens/antigens in body fluid

Question 4

2 differences between specific and nonspecific immune response

Answer 4

Nonspecific = immediate
Specific = time lag

Nonspecific = same for all pathogens
Specific= complementary to pathogen

Question 5

2 types of specific immune response

Answer 5

Cell mediated
Humoral

Question 6

Outline the process of cell mediated response

Answer 6

Complementary T helper cells bind to foreign antigen
Release cytokines that stimulate
a) clonal selection of complementary T lymphocyte cells become memory cells or trigger humoral response
b) clonal selection of cytotoxic T cells secrete enzymes to hydrolyse infected cells

Question 7

Outline the process of the humoral repsonse

Answer 7

Complementary TH cells bind to foreign antigen in antigen presenting T cells
Release cytokines that stimulate clonal selection of B lymphocytes with complimentary antibody to foreign antigens
B cells differentiate into plasma cells
Plasma cells secrete monoclonal antibodies with complementary variable region to antigen

Question 8

What is an antibody

Answer 8

Proteins secreted by plasma cells that is specific to an antigen
Quaternary structure
Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen
The rest of the molecule is the constant region

Question 9

How do antibodies lead to the 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 produces from a single clone of B cells

Question 11

What are memory cells

Answer 11

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

Question 12

Contrast the primary and secondary immune response

Answer 12

Secondary - 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

Random genetic mutation changes 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 tertiary structure
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, both can be natural or artificial
Passive N - antibodies in breastmilk/placenta
Passive A - anti-venom, needle stick injections
Active N - humoral response to infection
Active A - vaccination

Question 16

Contrast passive and active immmunity

Answer 16

Passive - no memory cells produces and 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 17

Explain the principles of vaccination

Answer 17

Vaccine contains dead/inactive form of pathogen
Triggers primary immune response
Memory cells are produced and remain in the bloodstream so secondary response is rapid and produces higher concentration of antibodies
Pathogen is usually 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

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 (RNA) and viral enzymes (integrase and reverse transcriptase) surrounded by capsid
Surrounded by viral envelope derived from host cell membrane
GP120 attachment proteins on surface

Question 21

How does HIV result in the symptoms of AIDS?

Answer 21

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

Question 22

Why are antibiotics ineffective against viruses

Answer 22

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 23

Suggest the clinical applications of monoclonal antibodies

Answer 23

Pregnancy tests by detecting HCG hormones in urine
Diagnostic procedures eg ELISA test
Targeted treatment by attaching drug to antibody so that it only binds to cells with abnormal antigen eg cancer cells due to specificity of tertiary structure of binding site

Question 24

Explain the principle of a direct ELISA test (detects antigen)

Answer 24

Monoclonal antibodies bind to bottom of test plate
Antigen molecules in sample bind to antibody. Rinse excess.
Mobile antibody with ‘reporter enzyme’ attached binds to antigens that are ‘fixed’ on the monoclonal antibodies. Rinse excess
Add substrate for reporter enzyme. Positive result = colour change

Question 25

Explain the principle of an indirect ELISA test (detects antibody)

Answer 25

Antigens bind to bottom of test plate
Antibodies in sample bind to antigen. Wash away excess
Secondary antibody with ‘reporter enzyme’ attached binds to primary antibodies from the sample
Add substrate for reporter enzyme. Positive result = colour change

Question 26

Suggest some ethical issues surrounding the use of monoclonal antibodies

Answer 26

Production involves animals
Drug trials against arthritis and leukaemia resulted in multiple organ failure