2.4 Cell recognition and the immune system

Question 1

What is an antigen?

Answer 1

Cell-surface molecule which stimulate immune response.

Usually (glyco)protein, sometime (glyco)lipds or polysaccharide

Immmune system recognises as “self” or “non-self” enabling identification of cells from other organisms of same species, pathogen, toxins and abnormal body cells

Question 2

How does phagocytosis destroy pathogens?

Answer 2

1. Phagocyte moves towards pathogen via chemotaxis
   2.phagocyte engulfs the pathogen via endocytosis to form a phagosome
   3.phagosome fuses with lysosome (phagolysome)
   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 antigens from pathogen on its surface (after hydrolysis in phagocytosis)

Enhancing recognition by T helper 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 and an immediate response
Specific (B and T lymphocytes) = complementary pathogen and has a time lag

Question 5

Name the two types of specific immune response

Answer 5

Cell mediated
Humoral

Question 6

Outline the process of the cell-mediated response

Answer 6

In cell mediated response Complementary T helper lymphocytes bind to foreign antigen on APC. Releasing cytokines that stimulate the clonal expansion of complementary T helper cells by rapid mitosis which become memory cells or trigger humoral response or the clonal expansion of cytotoxic T cells which secrete enzyme perforin to destroy infected cells

Question 7

Outline the process of the humoral response

Answer 7

1.Complementary T helper 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 8

What is an antibody

Answer 8

Proteins secreted by plasma cells, Contains a 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 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 produced from a single clone of B cells

Question 11

What are memory cells?

Answer 11

Specialised T helper/B cells produced from primary immune response,remaining in low levels in the blood. They are able to divide very rapidly by mitosis if organism encounters the same pathogen again

Question 12

Contrast the primary and secondary immune response

Answer 12

Secondary response

Faster rate of antibody production

Shorter time lag between exposed 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

Antigen variability is caused by
1.Random genetic mutation changes DNA base sequence
2.Results in different sequence of codons on mRNA
3.Different primary structure of antigens formed as H-bonds, ionic bonds and 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 so the individual not immune meaning that they can catch the disease more than once

Many varieties of a pathogen so 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: needle stick injections
Active natural: humoral response to infection
Active artificial: vaccination

Question 16

Contrast passive and active immunity

Question 17

Explain the principle 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 response is rapid and produces higher concentration of antibodies

4.pathogen is destroyed before it causes symptoms

Question 18

What is heard 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 weak immune systems

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 (2XRNA) 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

1, attachment proteins bind to complementary CD4 receptors on T helper cells

2. HIV particles replicate inside T helper cells, killing or damaging them
3. AIDS developes when there are too few T helper cells for the immune system to function
4. 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

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 24

Explain the principle of a direct ELISA test

Answer 24

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 antigen that are ‘fixed’ on the monoclonal antibodies. Rinse excess.
4. Add substrate for reporter enzyme. Positive result: colour change

Question 25

Explain the principle of an indirect ELISA test

Answer 25

Detects presence of an antibody against a specific antigen

1.antigen binds 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 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