Exam question: Self mark it

(a) (i) A disease-causing organism / bacterium; 1

(b)(i) Attracted by chemicals/antigensFormation of vesicle / phagosome;

lysosome fuse/hydrolytic enzymes digest Present Antigens on surface (APC) 2

(c) (i) Lysosome; 1

(ii) Contain hydrolytic enzymes;To break down / digest bacterium; 2

Specific Immunity

Non-Specific(Response is immediate

and the same for all pathogens)

SpecificResponse is slower and

specific to each pathogen

Physical Barrier

Phagocytosis Cell mediated response

T lymphocytes

Humoral response

B lymphocytes

Defence mechanisms

What are Anitgens?

An antigen is any part of an organism that is recognised as being non-self. By the immune system and stimulates the immune response.

(anti –antibody, gen-generator)

– Usually proteins or glycoproteins on the cell plasma membrane or cell wall of invading pathogen.

Specific ImmunityResponse: targets specific antigens

is slowerprovide long term immunity

•Involves lymphocytes which are WBC which circulate the body in blood and lymph.

•There are two types of lymphocyte both of which develop in bone marrow

- T cells – (Cell mediated response) - mature in thymus gland- B cells (humoral immune system) – mature in bone marrow

Both are produced in the stem cells of the bone marrow

Cell-mediated response

T lymphocytes (T cells)

T cells can distinguish and respond to foreign cells or APC by the antigens on their surfaces.

Receptors on T cells are specific to certain antigens

Cell mediated – acts inresponse to infected cells

1. Pathogens invade body cells or are taken in by phagocytosis

2. The phagocyte places antigens from the pathogen on its cell surface membrane becoming an Antigen Presenting Cell (APC)

3. Receptors on a specific helper T cell (TH cell) fit exactly onto these antigens

4. This attachment activates the T cells to divide rapidly by mitosis and form a clone of genetically identical cells (clonal expansion)

5. The cloned T cells:

a) develop into memory cells (T helper clones) that enable a rapid response to future infections by the same pathogen.

b) stimulate phagocytes to engulf pathogens by phagocytosis

c) stimulate B cells to divide and secrete their antibodies

d) activate cytotoxic T cells (TC cells) also known as killer T cells

Also produces cytokines (interleukins) that alert other T helper cells

How cytotoxic T cells kill infected cells

1. Cytotoxic T cells release proteins (cytokines) eg perforin2. Perforin attach to membranes of infected cells3. Makes holes in the cell membrane4. Cell lysis

Consolidation Define a pathogen

…………………………………………………………………….…………………....

(1 mark)

Explain the role of Antigen Presenting Cells (APC’s) in cell mediated immunity. ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(3 marks)

What feature of the T helper cell allows it to identify specific foreign antigens?………………………………………………………………………………………….

(2 marks)

Consolidation – self assess Define a pathogena disease causing microorganism;

…………………………………………………………………….…………………....

(1 mark)Explain the role of Antigen Presenting Cells (APC’s) in cell mediated immunity. APC’s are phagocytic cells/cells that perform phagocytosis;…………………………………………………………………………………………lytic enzymes break down pathogen/pathogen invades the APC;…………………………………………………………………………………………Presents specific antigen on cell surface;…………………………………………………………………………………………

(3 marks)What feature of the T helper cell allows it to identify specific foreign antigens?Specific receptors that are complementary/same fit/ exact fitto antigen on APC surface(2 marks)

Non-Specific(Response is immediate

and the same for all pathogens)

SpecificResponse is slower and

specific to each pathogen

Physical Barrier

Phagocytosis Cell mediated response

T lymphocytes

Humoral response

B lymphocytes

Defence mechanisms

Specific ImmunityResponse: targets specific antigens

is slowerprovide long term immunity

•Involves lymphocytes which are WBC which circulate the body in blood and lymph.

•There are two types of lymphocyte both of which develop in bone marrow

- T cells – (Cell mediated response) - mature in thymus gland- B cells (humoral immune system) – mature in bone marrow

Both are produced in the stem cells of the bone marrow

Humoral response

Humoral involves B cells that produce antibodies in response to antigens in the plasma

Antibodies are specific to complementary antigens

Each type of antibody is produced by a different type of B cell

What is an antibody?

• They are made up of 4 polypeptide chains (2x light 2x heavy)

• 2x antigen binding sites• Variable region/constant region• Disulphide bond

Antibodies are proteins with specific binding sites produced in the presence of a specific antigen

1) The antigens of an invading pathogen are taken up by the complementary B cell.

2) The B cell processes the antigens (via endocytosis) and presents them on its surface

3) T Helper Cells (already activated by cell mediated response) attaches to the processed antigen thereby activating the B cell

4) The B cell is now activated to divide by mitosis to give clones (clonal selection):•Plasma cell•Memory cells

5) The cloned plasma cells produce and secrete the specific antibodies (monoclonal antibodies) that exactly fit the antigen on the pathogen’s surface.These are:

Short lived but make up to 200 antibodies per second

6) The antibody attaches to antigens on the pathogen and destroys them by agglutination. This is the primary immune response

• Antibodies neutralise toxins and can stick pathogens together making it easier for phagocytes to engulf and destroy – act as labels

Agglutination

7) Some B cells develop onto memory cells . These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies . This is the secondary immune response

Memory Cells• Live longer than plasma cells – often for decades.• They do not directly produce antibodies but circulate in

the plama and tissue fluid. • When they encounter the same antigen later they can

divide rapidly and clone to produce more plasma and memory cells.

• Plasma cells then produce lots of anti bodies very quickly.

• Provide long term immunity

• Primary immune response: The first time a B cell comes into contact with a non-self antigen that is complementary to its cell receptors the production of sufficient antibody producing cells takes between 10-17 days – thus the person is likely to feel some symptoms.

• Secondary immune response: Memory cells which recognise the antigen produce plasma cells which release antibodies to fight the antigen. This takes 3-7 days – creating immunity.

Faster production of antibodies

Produce more antibodies

Symptoms may not be experienced

Consolidation – self assess Describe the function of a cytotoxic T cell

…………………………………………………………………….…………………....

(1 mark)Explain the importance of the memory B cells in immunity.………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(2 marks)Use knowledge of antigen-variability to explain why people can still become infected with a flu virus even though they have had this virus before. ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(3 marks)

Consolidation – self assess Describe the function of a cytotoxic T cell

Releases toxic chemical/perforin/substance/ into pathogenic cells (1 mark)

Explain the importance of the memory B cells in immunity.Produce antibodies when reinfection;Of the same antigen;

(2 marks)Use knowledge of antigen-variability to explain why people can still become infected with a flu virus even though they have had this virus before. Memory cells recognise the same antigens;Flu virus has many strains with many different antigens;Primary immune response is required;

(3 marks)