Cell Recognition And The Immune System (3.2.4) (A)

Question 1

What are pathogens?

Answer 1

• Microorganisms that cause disease, including bacteria, viruses and fungi

Question 2

What are non-specific responses?

Answer 2

• Immediate and the same for all pathogens
• They include physical barriers, such as the skin and phagocytosis

Question 3

What are specific responses?

Answer 3

• Slower but tailored to each pathogen
• Involve lymphocytes

Question 4

What are the two types of specific responses?

Answer 4

• Cellular/cell-mediated response
• Humoral response

Question 5

What do lymphocytes need to do to defend the body from invasion by foreign material?

Answer 5

• They need to be able to distinguish between self and non-self cells

Question 6

How do lymphocytes distinguish between self and non-self cells?

Answer 6

• They recognise specific proteins on the cell surface, which have unique structures that identify them as either self or non-self

Question 7

What are antigens?

Answer 7

• Foreign protein that stimulates an immune response

Question 8

What is antigenic variation/variability?

Answer 8

• When pathogens change their surface antigens, preventing the immune system from recognising the pathogen and making the immune response less effective during subsequent infections

Question 9

What is a phagocyte?

Answer 9

• A type of white blood cell which carries out phagocytosis

Question 10

What is phagocytosis?

Answer 10

• The engulfment of microorganisms by phagocytes

Question 11

What are the steps in phagocytosis?

Answer 11

• The phagocyte detects microbes by the chemicals they give off
• The microbe is engulfed by the phagocyte
• Phagosome forms which encloses the microbe in a vesicle
• Phagosome fuses with a lysosome, releasing lysozymes into the phagosome to digest the pathogen
• The lysozymes hydrolyse the pathogen
• Indigestible matter is discharged from the phagocyte (exocytosis)
• The phagocyte presents pathogen’s antigens on its cell-surface membrane

Question 12

What are antigen presenting cells?

Answer 12

• Cells that process and present foreign antigens on their surface, including phagocytes or cells from transplanted organs

Question 13

What do T cells do?

Answer 13

• Have receptors on their cell-surface membrane that bind to specific antigens
• These receptors are complementary to a single specific antigen

Question 14

How does the cellular response work?

Answer 14

• Involves helper T cells binding to antigens presented by antigen presenting cells

Question 15

What are the steps in the cellular response?

Answer 15

• A phagocyte engulfs the pathogen and destroys it by phagocytosis
• It then processes the pathogen’s antigens and presents them on its cell-surface membrane becoming an antigen presenting cell
• Helper T cells with the complementary receptors will bind to the antigen, activating the helper T cells
• After activation the helper T cells divide by mitosis forming a clone of genetically identical T cells with the same receptor
• Some stimulate B cells to divide and secrete antibodies
• Some stimulate phagocytes to perform more phagocytosis
• Some develop into cytotoxic T cells
• Some develop into memory cells, enabling a secondary response if a future infection occurs with the same pathogen

Question 16

What is the humoral response?

Answer 16

• The response of B lymphocytes to a foreign antigen

Question 17

How are B cells activated?

Answer 17

• When they bind to a specific antigen directly or to an antigen found on an antigen presenting cell

Question 18

What are the steps in the humoral response?

Answer 18

• The specific antigen binds to a complementary antibody on the surface of a B cell
• These B cells undergo clonal selection
• The clone of B cells differentiate into plasma cells and memory cells
• Plasma cells secrete antibodies specific to the antigen

Question 19

What is clonal selection?

Answer 19

• The process by which the specific B cell that binds to an antigen is stimulated to divide and build up its numbers

Question 20

What happens to the antibodies secreted by plasma B cells?

Answer 20

• The antibodies bind specifically to the antigens forming an antibody-antigen complex
• This stimulates processes leading to the destruction of the pathogen

Question 21

What is the difference between plasma cells and memory cells?

Answer 21

• Plasma cells secrete antibodies, usually in blood plasma and are involved in the primary immune response
• Memory cells circulate in the blood and tissue fluid and they’re involved in the secondary immune response

Question 22

What are antibodies?

Answer 22

• A protein specific to an antigen produced by B lymphocytes

Question 23

What is the basic structure of antibodies?

Answer 23

• Consists of four polypeptide chains: 2 heavy and 2 light chains, joined by disulfide bonds
• Each polypeptide chain consists of 2 regions

Question 24

Describe the regions of antibodies

Answer 24

• In the constant region the sequence of amino acids is the same in all molecules of the same type of antibody
• The variable region differs between antibodies and allows them to be specific to different antigens. The antigen-binding sites are formed here

Question 25

What is important about antigen-binding sites?

Answer 25

- These sites have a specific tertiary structure complementary to the structure of the antigen molecule which they attach to form an antibody-antigen complex

Question 26

What is agglutination?

Answer 26

- This refers to the clumping together of cells possessing the antigen which enhances their destruction by phagocytosis

Question 27

What are the steps in agglutination?

Answer 27

- An antibody molecule can use its two antigen-binding sites to attach to the same antigen present on two different cells - They join the cells together - As more antibody molecules attach more cells are linked together to form an agglutinated mass of cells which are then more easily destroyed by phagocytosis

Question 28

What is the stimulation of phagocytosis?

Answer 28

- One type of antibody attaches to the antigen on the surface of a pathogen and identifies it for destruction by phagocytes - Phagocytes have receptors in their cell-surface membrane that recognises the antibody and enable them to bind to, engulf and destroy the pathogen

Question 29

What is the primary response?

Answer 29

- Slower, taking several days to produce a significant concentration of antibodies - During this time, pathogens can reproduce and disease symptoms may appear

Question 30

What is the secondary response?

Answer 30

- If the same antigen is encountered again, memory B cells recognise it quickly - Memory B cells divide rapidly and develop into plasma cells - Plasma cells produce antibodies faster and in higher concentrations