2.4 Immune system

What is an antigen?

A foreign molecule that stimulates an immune response, leading to the production on an antibody

How are cells identified by the immune system?

Each type of cell has specific molecules on its surface that identify it

What type of cells can the immune system identify?

• Pathogens (viruses,bacteria)

• Cells from other organisms (organ transplants)

• Abnormal body cells (tumour, cancer)

• Toxins (poisons released by some bacteria)

What is the non-specific immune response?

Phagocytosis

What is the cellular response?

T lymphocytes forming clones which stimulate cytotoxic t cells, specific B cells and phagocytes

What is the humoral response?

B lymphocytes forming clones which differentiate into B plasma cells or B memory cells

Describe the process of phagocytosis/non-specific immune response

1. Phagocyte recognises foreign antigens on pathogen

2. Phagocyte engulfs pathogen by moving around it

3. Pathogen is contained in phagosome in cytoplasm of phagocyte

4. Lysosome fuses with phagosome and releases lysozymes

5. Lysozymes digest pathogen

6. Antigens presented on surface of phagocyte to stimulate an immune response

• Describe the process of the cellular response

1. T lymphocytes recognise antigens on surface of antugen presenting cells (infected cells, phagocytes, tumour cells, transplanted cells)

2. Specific T helper cells with complimentary receptors on the surface bind to antigens on antigen-presenting cell

3. This activates the T cell and causes it to divide by mitosis to form clones which stimulate cytotoxic T cells, specific B cells, phagocytes

What do cytotoxic T cells do?

Kill infected cells/tumour cells

Describe the process of the humoral response

Clonal selection -

1. Specific B lymphocyte with complimentary receptors on cell surface bind to antigen

2. This is stimulated by helper T cells

3. B lymphocyte divides rapidly by mitosis to form clones which differentiate into B plasma cells or B memory cells

What do B plasma cells do?

Secrete large amounts of antibodies

What do B memory cells do?

Remain in the blood for secondary immune response

What are antibodies?

Quaternary structure proteins secreted by B lymphocytes (eg plasma cells) in response to specific antigens

What do antibodies bind to and what does this form?

Antibodies bind to specific antigens, forming antigen-antibody complexes

Label the structure of an antibody

What is agglutination?

Antibodies bind to 2 pathogens at once causing them to clump together

How do antibodies lead to the destruction of pathogens?

1. Antibodies bind to antigens on pathogens, forming an antigen-antibody complex (specific tertiary structure so binding site/variable region binds to complementary antigen)

2. Each antibody binds to 2 pathogens at a time, causing agglutination (clumping) of pathogens

3. Antibodies attract phagocytes

4. Phagocytes bind to the antibodies and phagocytosis many pathogens are once

Explain the primary immune response

• Happens during first exposure to antigen

• Antibodies are produced slowly and at a lower concentration

• Takes time for specific B plasma cells to be stimulated to produce specific antibodies

• Memory cells are produced

Explain the secondary immune response

• Happens during second exposure to antigen

• Antibodies are produced faster and at a higher concentration

• B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies

What is a vaccine?

Injection of antigens from dead or inactive pathogens, stimulating the formation of memory cells

Explain how vaccines protect individuals from disease

1. Specific B lymphocyte with complimentary receptor binds to antigen

2. Specific T helper cell binds to antigen-presenting cell and stimulates B cell

3. B lymphocyte divides by mitosis to form clones which differentiate into B plasma cells which release antibodies or B memory cells

4. On secondary exposure to antigen, B plasma cells rapidly divide by mitosis to produce B plasma cells. These release antibodies faster and at a higher concentration

Explain how vaccines protect populations against disease

Herd immunity - large proportion of populations against vaccinated, reducing spread of pathogen

• Large proportion of population immune so do not become ill from infection

• Fewer infected people to pass pathogen on/unvaccinated people less likely to come in contact with someone with disease

Describe the differences between active and passive immunity

Active - Initial exposure to antigen (eg vaccine or primary infection)

Passive - No exposure to antigen

Active - Memory cells involved

Passive - No memory cells involved

Active - Antibodies produced and secreted by B plasma cells

Passive - Antibodies introduced from another organism (eg through breast milk)

Active - Slow so takes longer to develop

Passive - Faster acting

Active - Long term immunity as antibody can be produced in response to a specific antigen again

Passive - Short term immunity as antibody hydrolysed

Explain the effect of antigen variability on disease and disease prevention

• Antigens on pathogens change shape/tertiarystructure due to gene mutations

• No longer immune (from vaccines or prior infection) as B memory cells receptors can’t bind to changed antigen on secondary exposure and specific antibodies aren’t complimentary/can’t bind to changed antigen

Examples - yearly new flu vaccines

Label the structure of a HIV particle

Describe the replication of HIV in T-helper cells

1. HIV attachment proteins attach to receptors on helper T cell

2. Lipid envelope fuses with cell-surface membrane, releasing capsid into cell

3. Capsid uncoats, releasing RNA and reverse transcriptase

4. Reverse transcriptase converts viral RNA to DNA

5. Viral DNA inserted into helper T cell DNA

6. Viral proteins are produced

7. Virus particles assembled and released from cell

What does AIDS stand for?

Acquired immune deficiency syndrome

Explain how HIV causes the symptoms of AIDS