Cells (CELL RECOGNITION AND THE IMMUNE SYSTEM)

Question 1

Define antigen

Answer 1

An antigen is a substance foreign to the body (non self) that causes the production of antibodies from the body and causes an immune response

Question 2

Define antibody

Answer 2

A protein with a specific binding site (due to its specific tertiary structure) complimentary to a specific antigen, synthesised and stored by plasma cells

Question 3

Describe the structure of an antibody

Answer 3

Y shaped
Disulphide bridges
Variable region and constant region
At the 2 top ends of the Y is the specific complimentary antigen binding site

Question 4

What is the body’s first line of defence and give some examples

Answer 4

Natural barriers that reduce the risk of infection (often exterior defences)
Eg:
Skin and membranes
Ciliated epithelium (goblet cells secrete mucus) and trap microbes in inhaled air
Acid in stomach
Sweat contains antimicrobial agents
Blood clots
Lysozyme present in tears break down bacterial cells walls (contain murein)

Question 5

Describe the process of phagocytosis

Answer 5

The bacteria (pathogen) is attracted to the phagocyte using surrounding antigens
Pathogen gets engulfed by phagocyte which forms a phagocytic vesicle
Lysosomes fuse to vesicle
Lysozymes digest/ hydrolyse pathogen into monomers.
Antigens in bacteria surround the membrane to produce antigen presenting cell

Question 6

Describe the process of cellular mediated immune response

Answer 6

1) Phagocytosis - forms an antigen presenting cell with antigens attached to the cell surface membrane of the phagocyte
2) Specific complimentary helper T cell binds to antigens on antigen presenting cell
3) Helper T cell undergoes clonal selection and divides by mitosis
4) This activates B cells which produces antibodies or cytotoxic T cells or memory cells

Question 7

Describe what cytotoxic T cells do

Answer 7

Cytotoxic T cells bind to infected cell
Produce perforin which makes holes in the cell surface membrane
Infected cell lyses (dies)

Question 8

Describe the process of humoral immunity

Answer 8

Antigens from pathogen bind to specific complimentary receptors on a B cell to form antigen presenting cell
Specific helper T helper cell activates B cell
B cell undergoes clonal selection and divides by mitosis to form memory B cells and plasma cells
Plasma cell releases antibodies and causes the agglutination of bacteria cells which causes bacteria cells to stick together and act as markers to signal phagocytes to engulf bacterial cell
Apoptosis

Question 9

Describe how a pregnancy test works

Question 10

Define what is meant by a monoclonal antibody

Answer 10

a single type of antibody produced by a single clone of plasma cells

Question 11

Give some uses of monoclonal antibody’s

Answer 11

Treat cancers, pregnancy tests

Question 12

Describe the process of direct monoclonal antibody therapy to treat cancer

Answer 12

• Monoclonal antibody is produced that is complimentary to the antigens on cancerous cell
• When antibody is given to patient, antibody attaches to antigen, preventing chemicals that stimulate mitosis from binding to it

Question 13

Describe the process of indirect monoclonal antibody therapy to treat cancer

Answer 13

• A cytotoxic drug (one that kills cancer) is attached to the monoclonal antibody. This antibody has a complimentary shape to the antigens on the cancer cells
• When antibodies attach to antigen, enzyme is delivered directly to cancer cells and doesn’t kill it
  –> less side effects and doesn’t damage surrounding cells
  –> known as a “bullet drug” as its delivered straight to cancer cell

Question 14

Explain how HIV affects the production of antibodies when AIDS develops in a person

Answer 14

Less antibodies are produced as HIV destroys helper T cells so less B cells can be activated

Question 15

Explain how HIV replicates

Answer 15

HIV can only replicate inside an organism
* HIV attachment proteins bind to specific receptors called CD4 on helper T cell
* HIV capsule fuses to helper T cell membrane, injecting its RNA and reverse transcriptase enzyme
*RNA is converted into DNA through the reverse transcriptase enzyme
* DNA moves into nucleus of T cell
* DNA is transcribed into HIV mRNA.
* HIV mRNA is translated into new HIV proteins for assembly into viral particles

Question 16

Explain how a vaccine works

Answer 16

Vaccine contains antigens of a pathogen displayed on antigen presenting cell
Antigens bind to specific complementary receptors on B cells
Helper T cell activates B cell
B cell undergoes clonal selection by mitosis to form plasma cells and memory cells
B cells produce antibodies

Vaccine contains antigen /dead/inactive pathogen;
Specific helper T cell stimulates B cell specific to antigen
B cell clones/divides by mitosis;
Plasma cells release antibodies;
Memory cells produced meaning higher concentration of antibodies/antibodies produced faster in secondary response/on infection with the actual pathogen;

Question 17

Describe the ELISA tests and what it tests for

Answer 17

Antigens of the virus (eg: HIV antigens) are attached to the bottom of a well in a well plate
Sample of blood added with potential complimentary antibodies which will bind to HIV antigen.
Wash to remove any unbound antibody
Add a second antibody which is attached to a specific enzyme. This antibody is complimentary to the first antibody and so will only bind if the first antibody is present and bound to the HIV antigen
Wash again
A colour change will occur as the enzyme reacts
Colour change = positive result

Question 18

What does a stronger colour change in the ELISA test show

Answer 18

Stronger colour change = higher concentration of antibodies

Question 19

Describe some of the ethical considerations raised with monoclonal antibodies

Answer 19

–> Creating monoclonal antibodies requires mice to be injected in their spleen causing them to die for the betterment of humans

Question 20

Describe active immunity

Answer 20

Active immunity:
–> Direct contact with the pathogen/ antigen is necessary
–> Antibodies secreted by plasma cells
–> Involves immune response and memory cells
–> Immunity is long term, but takes time to develop as antibody is produced in response to antigen

Question 21

Describe passive immunity

Answer 21

–> No direct contact with antigen or pathogen.
–> Antibodies are introduced from an outside source
–> No memory cells formed and so no lasting immunity.
–> Immediate protection (fast acting) , but short term as antibodies are broken down