Cell recognition and immune response

Question 1

List the defence mechanisms that are:

a) non-specific (2)
b) specific (2)

Answer 1

a) - physical barrier
- Phagocytosis

B) - cell-mediated response
- humoral response

Question 2

Describe the process of Phagocytosis (5)

Answer 2

1. Phagocyte is attracted to chemical products of pathogen, moving towards the pathogen along a concentration gradient
2. Phagocytes have several receptors that bind to chemicals found on the surface of pathogen
3. Phagocyte engulfs pathogen forming phagosome
4. Lysosomes migrate to phagosome releasing lysozymes that hydrolyse the cell walls of pathogen killing it
5. Hydrolysis products of pathogen absorbed into cytoplasm of phagocyte and pathogenic antigens displayed on cell membrane (APC)

Question 3

Define antigen (1)

Answer 3

Any part of an organism or substance (usually protein) that is recognised as non-self stimulating an immune response

Question 4

Describe cell-mediated immunity (5)

Answer 4

1. Pathogens invade body cell or taken in via phagocytosis
2. Phagocyte/body cell presents pathogenic antigen on own cell membrane (APC)
3. Specific receptors of helper T cell with a 3D tertiary structure complementary to antigen bind
4. Binding activates helper T cell to divide rapidly by mitosis ( clonal expansion)
5. Divide into:
   a) memory cells for rapid future response
   b) stimulate phagocytes to engulf the pathogen
   c) stimulate B cells to divide and secrete antibody
   d) Activate cytotoxic T cells that kill infected cells via making holes in their cell membranes

Question 5

How do cytotoxic T cells function? (2)

Answer 5

1. Cytotoxic T cells produce a protein Perforin that makes holes in cell membrane
2. Holes mean that cell membrane becomes freely permeable to all substances and cell dies as a result

Question 6

Explain the Humoral Response (5)

Answer 6

1. B cell processes antigens from pathogen or APC and presents the antigen on its own cell membrane
2. Activated helper T-cells bind to the antigen on B cell via complementary 3d tertiary protein structure
3. B cell stimulated to divide rapidly via mitosis (clonal expansion) to form plasma and memory cells
4. Plasma cells secréte antibodies with receptors complementary to the specific antigen of pathogen. Antibodies aid in the destruction of pathogen in primary immune response
5. Memory cells remain in blood stream and when in contact with antigen again, divide into plasma and memory cells for a faster secondary immune response

Question 7

What are antibodies? (1)

Answer 7

1. Antibodies are proteins which bind to antigens to form an antigen-antibody complex which aids in the destruction of pathogens

Question 8

How do antibodies help fight disease? (4)

Answer 8

1. Agglutinisation - antibodies bind to antigens on multiple pathogens clumping them together. This makes it easier for phagocytes to engulf more pathogens at a time
2. Neutralising toxins - some antibodies neutralise toxins released by pathogens
3. Prevent viruses from entering cell - antibodies bind to viral proteins preventing them from binding to surface of host cell and entering
4. Antibodies serve as marker that stimulate phagocytes to engulf the bacterial cell

Question 9

Explain the structure of an antibody (3)

Answer 9

1. Made up of 4 polypeptide chains, 2 heavy, and 2 light
2. The variable region is a specific binding site to for the antigen-antibody complex
3. Rest of antibody is the constant region

Question 10

Explain how monoclonal antibodies are formed (6)

Answer 10

1. Mouse injected with specific pathogen
2. Mouse produces an immune response and Specific B lymphocytes produce antibodies
3. B-cells are combined with tumour cells to form hybridomas
4. Hybridomas then screened to ensure production of desired antibody
5. Hybridomas are capable of producing antibodies and dividing
6. Monoclonal (came from one cell) antibodies harvested

Question 11

Explain how direct monoclonal antibody therapy is used to treat cancer (3)

Answer 11

1. Monoclonal antibodies are produced that are specific to antigen on cancer cells
2. The monoclonal antibodies are given to patients and for antigen-antibody complexes with the cancer cells
3. By attaching to the surface of cancer cells, they block the chemical signals that stimulate uncontrolled growth

Question 12

Explain how an ELISA test works (6)

Answer 12

1. A specific monoclonal antibody (mAb-1) is attached to a well
2. A test solution is added and, if present, molecules of targeted antigen bind to antibodies
3. Test solution is washed away, leaving only bound antigens
4. A second different monoclonal antibody (mAb-2) with a reporter enzyme attached is added and binds with antigens on molecule already bound to first antibody
5. Contents washed away again, only leaving bound molecules
6. The reporter’s enzyme substrate is added. If reporter enzyme present, it catalyses a colour change indicating the test antigen was present

Question 13

Explain the different types of immunity (4)

Answer 13

1. Natural active - results when an individual is infected by a disease and the body’s immune system produces its own antibodies
2. Artificial active - Vaccination that introduces pathogenic material that stimulates an individuals immune response producing antibodies
3. Natural passive - antibodies obtained externally such as of a foetus from its mother
4. Artificial passive - Direct introduction of antibodies into individual such as anti-venom

Question 14

Explain the features of a successful vaccination programme (5)

Answer 14

1. Must be economically available in sufficient quantities to immunise most of the vulnerable population
2. Few side-effects which could discourage individuals
3. Means of producing, storing and transporting vaccine
4. Means of administering vaccine properly at appropriate time. Trained individuals
5. Must be possible to vaccinate vast majority of vulnerable population to produce herd immunity

Question 15

Explain herd immunity and how it arises (4)

Answer 15

1. It arises when a sufficiently large population of the population have been vaccinated making it difficult for pathogen to spread within the population
2. Herd immunity is important as it is never possible to vaccinate the entire population (babies have undeveloped immune systems and people with compromised immune systems)
3. As pathogens are passed from person to person, it decreases the chance of infection, protecting vulnerable individuals
4. To achieve herd immunity, it is best to vaccinate everyone at one time so that at a certain period transmission of pathogen is interrupted

Question 16

Explain why vaccination may not eliminate a disease (6)

Answer 16

1. Vaccination fails to induce immunity in certain individuals with defective immune systems
2. Individuals may develop disease immediately after vaccination when immunity levels are not high enough. Can then spread pathogen
3. Pathogen may mutate frequently, suddenly changing its antigens. Antigenic variability occurs in influenza virus
4. May be so many varieties of a particular pathogen that it is impossible to develop a vaccine for all of them
5. Certain pathogens ‘hide’ from the body’s immune system such as by concealing in cells
6. Individuals may reject vaccine for religious, ethical or medical reasons

Question 17

Explain how HIV (retro)virus replicates (6)

Answer 17

1. HIV binds to receptors on T-helper cell
2. Protein capsid fuses with cell membrane releasing RNA and enzymes into T-helper cell
3. HIV reverse transcriptase convert virus’s RNA to DNA
4. Newly made virus DNA is inserted into cell’s DNA
5. HIV DNA creates mRNA that codes for viral proteins and RNA
6. HIV particle then breaks away from helper-T cell with a piece of cell membrane forming the lipid envelope

Question 18

Explain how HIV causes the symptoms of AIDS (4)

Answer 18

1. HIV virus attack helper T-cells killing or interfering with their functioning
2. Insufficient number of helper T-cells means that B cells cannot be stimulated to produce antibodies (humoral response) or cytotoxic T cells (cell mediated)
3. Memory B cells may also be infected and destroyed
4. Therefore, the body is unable to produce an adequate immune response and so infection by a secondary pathogen can lead to death

Question 19

Explain why antibiotics are ineffective on viral diseases (4)

Answer 19

1. Antibiotics weaken the murein cell wall of bacteria, causing them to burst as water flows in by osmosis
2. Virus’s lack their own metabolic pathways and cell structures that an antibiotic can potentially disrupt
3. Virus’s also have a protein coat and do not have sites where antibiotics can work
4. Antibiotics cannot reach virus’s when they are in host cells