10: Disease & Immunity

Question 1

What are 2 ways Pathogens cause disease?

Answer 1

• damaging host cells (viruses)
• releasing toxins (bacteria)

Question 2

How does Cell Recognition work?

Answer 2

• all cells have specific antigens on their cell-surface membrane which are used in cell recognition
• these molecules are specific to each organism, so the immune system can identify non-self (foreign) molecules

Question 3

What is an Antigen?

Answer 3

• non-self protein that stimulates an immune response (production of antibodies)

Question 4

What is the Non-Specific Immune Response?

Answer 4

• Phagocytosis

Question 5

Describe the process of Phagocytosis:

Answer 5

1. phagocytes identify non-self molecule and bind to antigen on pathogen
2. phagocytes engulf these pathogens by endocytosis
3. they are enclosed into a vesicle
4. lysosomes then fuse with vesicle
5. lysosomes contain lysozymes which contain hydrolytic enzymes to hydrolyse bacterial cell walls
6. phagocyte then digests the pathogen, by hydrolysis of its MOLECULES
7. phagocyte places antigen from pathogen on its cell-surface membrane, becoming an antigen presenting cell

Question 6

What are the 2 Specific Immune Responses?

Answer 6

• cellular response
• humoral response

Question 7

Describe the Cellular Response:

Answer 7

1. A t-helper cell containing a specific complementary receptor binds to an antigen on the antigen presenting cell
2. this then activates the T-helper cell to divide rapidly by mitosis, and differentiate into genetically identical cells
   - some of which are memory cells, (for reinfection)
   - some of which are T-helper cells
3. T-helper cells release specific cytokines to activate white blood cells e.g B cells and Cytotoxic T-cells
4. These cytotoxic T-cells then destroy infected body cells, by binding to the same complementary antigens to their receptors, causing them to release chemicals such as perforin, leading to holes in membrane of infected cells, leading to destruction

Question 8

Describe the Humoral Response:

Answer 8

1. receptors on the cell membrane on specific B-cells binds to antigen from antigen presenting cell
2. once the B-cell is complementary to the specific antigen, it is activated by the T-helper cells
3. through the use of cytokines, clonal selection occurs: B-cells divide rapidly (by mitosis) to form a clone
4. these clones differentiate into:
   - plasma cells
   - memory B-cells
5. the plasma cells produce and secrete antibodies, which bind to complementary non-self antigens leading to destruction of pathogens (agglutination)
   This is the primary response

Question 9

Describe the Secondary Response (Humoral):

Answer 9

If B-memory cells encounter pathogen at later date (with same specific antigen):
- they divide rapidly by mitosis
- produce a larger number of plasma + memory cells, for higher antibody secretion for faster agglutination

Question 10

Why is the Secondary response much faster than the Primary response?

Answer 10

• primary response: it takes time for the specific B-lymphocyte to be activated, to then clone itself, produce plasma cells, antibodies and memory cells
• secondary response: there are already memory cells against the specific antigen, so they can divide much more quickly by mitosis to produce larger number of plasma cells, therefore more specific antibodies at a faster rate (clonal selection faster), therefore quicker agglutination

Question 11

What is Antigenic Variability?
Why is this so?

Answer 11

• antigens on the surface of each strain are different
• this arises due to mutations in genes, causing them to code for antigens with different tertiary structure and shapes meaning antibodies is no longer complementary and can no longer bind and form antigen-antibody complexes

Question 12

What are Antibodies?
What do they do?

Answer 12

• antibodies are proteins synthesised by B cells
• they react with antigens with complementary shapes, forming antigen-antibody complexes
• antibodies are very specific

Question 13

What are Monoclonal Antibodies?

Answer 13

• produced by the same B cell/plasma cell
• have the same tertiary structure and therefore bind for one specific antigen

Question 14

What is the ELIZA test?

Answer 14

1. container coated with monoclonal antibodies
2. sample to be tested is added, and if specific antigen is present then antigen would bind to antibodies
3. a SECOND antibody, with an enzyme attached, is then added which binds to antigen
4. if antigen is present a substrate binds to the enzyme which is then hydrolysed producing a colour change

Question 15

ELIZA Test:
What would happen if no Antigens present?
Why is it important to wash after second antibody added?
How do you work out how much antigen is present?

Answer 15

• 2nd antigen with enzyme attached wouldn’t bind, therefore no colour change as enzyme would wash away
• remove any unbinded antibodies to prevent false positive
• more antigen present, more antigen-antibody complexes form, so more enzyme-substrate complexes form, therefore stronger colour change

Question 16

Describe how HIV works:

Answer 16

1. attachment proteins of virus bind to complementary receptors on the surface of T-helper cells
2. capsid- containing genetic material and an enzyme, reverse transcriptase, is released into T-helper cell
3. inside the cell reverse transcriptase and RNA are released
4. reverse transcriptase is used to make a complementary strand of DNA using viral RNA as template
5. double stranded DNA is made, and incorporated into the T-cells own DNA
6. new viral DNA used to make new viral proteins, which causes new HIV viruses to bud from one T- helper cells and infect others.

Question 17

How does HIV lead to AIDS?

Answer 17

• replication + release of HIV from T-helper cells leads to destruction of cells, so number of T-helper cells decrease
• therefore, fewer B cells, cytotoxic T-cells and phagocytes are activated
• therefore, fewer plasma cells and antibodies are made so cytotoxic T-cells and plasma cells can destroy infected cells
• so individual becomes more susceptible to other infections

Question 18

How is HIV treated?

Answer 18

• using a series of antiviral drugs, which often have to be changed as the virus has a high mutation rate and becomes resistant to them
• not antibiotics as viruses have no cell structure and are non-living