Immune system

Question 1

What is a pathogen?

Answer 1

A harmful microorganism that causes a disease

Question 2

What is an antibody?

Answer 2

A protein produced by white blood cells (plasma cells) against specific antigens

Question 3

What is an antigen?

Answer 3

A protein marker on a pathogen that initiates an immune response and the production of antibodies

Question 4

What is a lymphocyte?

Answer 4

A white blood cell that produces antibodies or antitoxins

Question 5

What are non specific defence mechanisms?

Answer 5

Response is immediate and the same for all pathogens

• tears - contain lysosome enzymes which kill pathogens
• mucus in nose - traps pathogens and cilia woft away mucus
• phagocytosis - white blood cells engulf and destroy pathogens

Question 6

What are specific defence mechanisms?

Answer 6

Response is slower and specific to each pathogen

• cell-mediated response - T lymphocytes
• humoral response - B lymphocytes

Question 7

Why are antigens important?

Answer 7

white blood cells must be able to distinguish the body’s own cells from those that are foreign. If they could not do this, the lymphocytes would destroy the organisms own tissues

Question 8

What is the structure of an antibody?

Answer 8

Quaternary structure - 2 ‘light chains’ held together by disulphide bridges, 2 longer ‘heavy chains.

‘Variable regions’ containing an antigen-binding site - have a specific tertiary structure complementary to a specific antigen

The rest of the antibody is the ‘constant region’ - same in all antibodies

Question 9

Describe the process of phagocytosis

Answer 9

• A chemical is released by the pathogen which attracts a phagocyte (chemotaxis)
• The antigens on the surface of the pathogen “dock” with receptors on the phagocyte
• The phagocyte envelops the pathogen by changing the shape of its membrane and surrounding the pathogen in a layer of membrane to form a phagosome
• The phagosome fuses with a lysosome (phagolysosome) and lysosomes release lysozyme enzyme which destroys the pathogen
• Bacteria is hydrolysed and debris is excreted out of the phagocyte by exocytosis

Question 10

Where do B lymphocytes mature and what do they do?

Answer 10

Mature in the bone marrow

• involved in humeral immunity
• produce antibodies
• respond to foreign material OUTSIDE body cells
• respond to bacteria and viruses

Question 11

Where do T lymphocytes mature and what do they do?

Answer 11

Mature in the thymus gland

• involved in cell-mediated immunity
• respond to foreign material INSIDE body cells
• respond to own cells altered by viruses or cancer and to transplanted tissues

Question 12

Where is lymph contained in the body?

Answer 12

lymph vessels and lymph glands

Question 13

What are antigen presenting cells?

Answer 13

A type of phagocyte that has engulfed a pathogen and is now presenting antigens from the destroyed pathogen on its surface.

Question 14

How do T cells respond to foreign antibodies?

Answer 14

• Antigen presenting cells will travel from the site of infection to a gland in the lymphatic system to search for a complementary T lymphocyte
• Once the correct T lymphocyte is located, it will divide by mitosis and is cloned by clonal selection
• As they are cloned, they become specialised. Some become cytotoxic T cells which will leave the lymph gland and travel in the bloodstream to the site of infection, where they release toxins (called perforins) which cause holes to form in the membrane of pathogens and lead to their destruction
• Other T lymphocytes can become T helper cells. These cells go to the site of infection and attach to pathogens. They release chemicals called cytokines which attract other white blood cells to the area to fight against the pathogen
• Either type of T cell can become a memory cell once the pathogen is destroyed. This enables the antigen to be recognised more quickly if a second infection occurs, and clonal selection will be much more rapid

Question 15

How do B cells respond to foreign antibodies?

Answer 15

• antigens of pathogens are taken up by B cells, B cells then process these antigens and present them on its surface
• complementary T lymphocytes will bind to the B cells therefore activating them
• B cells divide into identical plasma B cells
• plasma B cells secrete antibodies which move to the site of infection through the bloodstream, and are complementary to antigens on the pathogen, so attach to and destroy it
• they will also produce memory cells which create a quicker response upon a second infection

Question 16

How do antibodies lead to the destruction of pathogens?

Answer 16

Formation of antigen-antibody complex which results in agglutination - a mass of pathogens surrounded by antibodies forming a large mass. This makes it easier for a phagocyte to locate and ingest because they release more chemicals

Question 17

What are monoclonal antibodies?

Answer 17

antibodies produced from 1 hybridoma cell that has been cloned by clonal expansion

Question 18

What are monoclonal antibodies used for?

Answer 18

• pregnancy tests
• diagnostic tests eg. covid tests
• medicine
• treatment for infection eg. malaria

Question 19

Contrast the primary and secondary immune response

Answer 19

• faster rate of antibody production
• shorter lag time between exposure and antibody production
• higher concentration of antibodies

Question 20

What causes antigen variability?

Answer 20

• random genetic mutation changes DNA base sequence
• results in different sequence of codons on mRNA
• different primary structure meaning that ionic, hydrogen and disulphide bonds form in different places in the tertiary structure
• different shape of antigen

Question 21

How are monoclonal antibodies produced?

Answer 21

antibody producing lymphocytes in mice are fused with tumour cells from humans (hybridoma) to rapidly reproduce, to produce large quantities of monoclonal antibodies

Question 22

Contrast passive and active immunity

Answer 22

Passive:
- no memory cells produced
- immediate
- antibodies from external source

Active:
- memory cells produced
- time lag
- lymphocytes produce antibodies

Question 23

What are the types of vaccination?

Answer 23

• attenuated vaccine - an altered pathogen that is less violent than usual pathogenic strains eg. flu jab
• dead microorganisms - bacteria or viruses are killed but still maintain some of their antigen structures eg. polio
• isolated antigens - only antigens or toxins are injected eg. tetanus

Question 24

How do vaccinations prevent you from developing a disease?

Answer 24

• vaccine contains antigens
• antigens are displayed on antigen presenting cells
• specific T helper cells detect antigen and stimulate B cells
• B cells divide by mitosis
• Plasma cells release antibodies
• B cells become memory cells and produce antibodies faster during secondary exposure

Question 25

What is herd immunity?

Answer 25

Vaccinating a large proportion of the population reduces available carriers of pathogens, which protects those who have not been vaccinated eg. people with weakened immune systems

Question 26

What are factors to consider for a successful vaccination programme?

Answer 26

• large quantities of vaccine available at a low cost
• few side effects
• vaccine can be produced, stored and transported hygienically
• trained medical staff available
• all of the population must be vaccinated
• education of population

Question 27

How does HIV effect the immune system?

Answer 27

the host cell for HIV is a T helper cell

• HIV enters the body through body fluids eg. blood/semen
• HIV enters T helper cells, releasing its RNA and reverse transcriptase into the cytoplasm
• reverse transcriptase copies viral RNA strand into a double stranded DNA
• viral DNA moves into nucleus and is integrated into host cell DNA
• virus DNA becomes active and new RNA virus particles are made
• RNA is translated by ribosomes into viral proteins
• helper T cells die when virus particles are released. These particles infect more cells

Question 28

What is the structure of HIV?

Answer 28

• genetic material (2x RNA) and viral enzymes (reverse transcriptase and integrase) surrounded by capsid
• surrounded by a lipid membrane and glycoproteins to help fuse with host cells surface membrane
• attachment proteins which bind to specific receptor molecules on the surface of a specific host cell

Question 29

How is HIV spread?

Answer 29

• unprotected sex
• sharing needles
• during pregnancy from mother to baby
• infected blood products

Question 30

Why are antibiotics ineffective against viruses?

Answer 30

• antibiotics often work by damaging murein cell walls - viruses have no cell wall
• viruses replicate inside host cells - difficult to destroy them without destroying normal body cells
• non-living so hard to treat

Question 31

How does the ELISA test work?

Answer 31

• monoclonal antibodies bind to bottom of test plate
• antigen molecules in sample bind to specific antibody. Rinse excess
• antibodies with an enzyme attached binds to antigens fixed on monoclonal antibodies. Rinse to avoid false positive
• add substrate for enzyme - substrate is broken down leading to colour change.

Question 32

What is clonal expansion?

Answer 32

B cells divide into identical cells that produce antibodies

Question 33

What is active immunity?

Answer 33

when the immune system has created its own antibodies

Question 34

What is passive immunity?

Answer 34

where an individual receives antibodies from and external source, antibodies are not made by the individual.