U3 Immunity: Immune response

Question 1

Define specific defences

Answer 1

Defences part of our immune system directed towards a particular pathogen

Question 2

Define immune system

Answer 2

A system composed of cells and proteins that protect against foreign organisms, alien chemicals, cancerous cells and abnormal cells

Question 3

What are non-specific immune cells and specific immune cells?

Answer 3

Non-specific immune cells:
Phagocytes, which engulf and digest microorganisms and cell debris

Specific immune cells:
B-cells and T-cells which only provide protection against specific microorganisms or disease-causing substances

Question 4

What is an immune response?

Answer 4

A response triggerd by foreign substances or microorganisms entering the body which helps deal with the invasion and restores the internal environment to its normal condition

Question 5

Compare and contrast B-cell and T-cells.

Answer 5

Similarities:
- Both types of white blood cells (lymphocytes)
- Both are produced in bone barrow
- Both reside in lymphoid tissue (lymph nodes)

Only B-cells:
- Develop into plasma cells that produce antibodies or develop into memory cells
- Mature in bone marrow
- Destroy pathogens by producing antibodies

Only T-cells:
- Differentiate into different kinds of cells involved in cell-mediated immunity
- Mature in thymus
- Destroy pathogens with cytotoxic T-cells, which destroy infected cells

Question 6

Define antigen

Answer 6

Any substance capable of causing a specific immune response

Question 7

What are examples of antigens?

Answer 7

• Large molecules (proteins, carbohydrates, lipids, nucleic acids)
• Whole microorganisms (bacterial cells)
• Parts of pathogens (parts of bacteria, virus particles)
• Molecules on cells (blood cells)
• Allergens (pollen grains, egg whites)
• Toxins

Question 8

What are the two types of antigens?

Answer 8

• Self-antigens: large molecules produced in a person’s own body that don’t cause an immune response
• Non-self antigens: foreign compounds that do trigger an immune response

Question 9

Define antibody

Answer 9

A Y-shaped specialised protein produced by plasma cells in response to a non-self antigen

Question 10

What is an antigen-antibody complex and how do they occur?

Answer 10

• A compound formed when an antibody combines with an antigen
• Antigen molecules have specific active sites with a particular shape which an antibody has a complementary shape to, allowing the molecules to fit together
• Each antibody can combine with only one particular antigen

Question 11

What methods do antibodies use to protect the body?

Answer 11

• Inactivating foreign enzymes or toxins by combining with them or inhibiting their reaction with other cells or compounds
• Binding to the surface of viruses to prevent them from entering cells
• Coating bacteria so they are consumed easier by phagocytes
• Causing particles of bacteria, viruses or foreign blood cells to clump together (process called agglutination)
• Dissolving organisms
• Reacting with soluble substances to make them insoluble and therefore more easily consumed by phagocytes

Question 12

What are antigen-presting cells?

Answer 12

Phagocytic cells that digest pathogens and present antigens to lymphocytes

• Dendritic cells, macrophages, undifferentiated B-cells

Question 13

What is the antigen-presenting cell process?

Answer 13

1. Antigen-presenting cell detects the presence of a non-self antigen
2. The cell engulfs the pathogen
3. The cell digests the pathogen, producing small fragments that move to the surface of the cell
4. The cell presents the antigen to the lymphocytes

Question 14

What are the two types of immune responses?

Answer 14

• Antibody-mediated immunity (humoral response)
• Cell-mediated immunity

Question 15

What are the steps of antibody-mediated immunity (humoral response)?

Answer 15

1. Lymphoid tissue contains thousands of types of B-cells which each have a receptor for a particular antigen that it’s capable of responding to
2. When an antigen-presenting cell presents an antigen to specific B-cells, the B-cells are activated
3. Cytokines (small proteins released in response to antigens that act as messengers in the immune response) are released by helper T-cells in response to the antigen
4. The cytokines cause the helper T-cells to clone themselves to release different cytokines which activate the B-cells
5. When the B-cells are activated, they enlarge and divide into groups of cells called a clone, which share the same genetic characteristics
6. Most of the clone becomes plasma cells, which secrete the specific antibody capable of attaching to the active site of the antigen
7. The antibodies circulate in the blood, lymph and extracellular fluid to reach the site of the invasion of microorganisms or foreign material
8. Any remaining B-cells of the clone become memory cells (a type of cell that recognises an antigen to which the body has previously been exposed)
9. The memory cells spread to all body tissues to allow the response to occur more rapidly if the antigen enters the body again

Question 16

What are the steps of the cell-mediated response?

Answer 16

1. When a foreign antigen enters the body, antigen-presenting cells present the antigen to T-cells in the lymphoid tissue, which become activated or sensitised
2. The sensitised T-cells enlarge and divide, creating a clone (a group of identical T-cells)
3. Some cells of the clone remain in the lymphoid tissue as memory cells, which are able to quickly recognise the antigen if it invades again, allowing a much faster immune response in subsequent infections
4. Most T-cells become cytotoxic/killer T-cells or helper T-cells and migrate to the site of infection

Cytotoxic/killer T-cells attach to antigens and secrete a chemical that destroys it

Helper T-cells bind to antigens on antigen-presenting cells, stimulating the secretion of cytokines which:
- Intensify the immune response
- Attract lymphocytes to the infection site, which become sensitised and activated
- Attract macrophages to the infection site so they can destroy the antigens by phagocytosis
- Intensify the phagocytic activity of the macrophages
- Promote the action of killer T-cells

5.
Suppressor T-cells act when immune activity becomes recessive, or the infection has been dealt with. They release substances that inhibit T-cell and B-cell activity, slowing down the immune response.