Cell recognition and the Immune System

Question 1

What are the 2 defence mechanisms in the body?

Answer 1

Non-specific:
- They are immediate and are the same for all pathogens.
- They include physical barriers and phagocytosis since the phagocyte detects any non-self cell and destroys it.

Specific:
- They are slower and specific for each pathogen.
- They include lymphocytes in the cell mediated and humoral response.

Question 2

What is an antigen?

Answer 2

A molecule that triggers an immune response by lymphocytes.

Question 3

How is a lymphocyte able to distinguish between a self and non-self cell?

Answer 3

The proteins on the surface of the cell can be specific and have different tertiary structure.

Question 4

What is a phagocyte?

Answer 4

A type of white blood cell called a macrophage that carries out phagocytosis

Question 5

Where are phagocytes found?

Answer 5

In the blood and tissues

Question 6

What is phagocytosis?

Answer 6

The process where cells engulf pathogens to form a vesicle or a vacuole.

Question 7

Describe the process of phagocytosis.

Answer 7

1. Phagocytes in the blood and tissue will be attracted to any chemical debris released by pathogens and move towards them
2. The receptor binding points on the phagocytes will attach to the antigens on the pathogen via these receptors.
3. The phagocyte changes shape to engulf the pathogen and contains it inside a phagosome vesicle
4. A lysosome within the phagocyte will fuse with the phagosome and release its contents
5. The lysozyme enzyme is released into the phagosome which hydrolyses the pathogen. This destroys the pathogen
6. The soluble products are absorbed and used by the phagocyte. The antigen is placed on the cell surface membrane so the phagocyte becomes an antigen presenting cell.

Question 8

What are T lymphocytes?

Answer 8

White blood cells involved in the specific immune system

Question 9

Where are T lymphocytes made?

Answer 9

Made in the bone marrow but mature in the thymus

Question 10

What are the two types of lymphocytes?

Answer 10

T lymphocytes and B lymphocytes

Question 11

Where are B lymphocytes made?

Answer 11

Made and mature in the bone marrow

Question 12

What processes are B and T lymphocytes associated in?

Answer 12

T lymphocytes - Cellular response
B lymphocytes - Humoral response

Question 13

What is an antigen presenting cell?

Answer 13

Any cell that presents a non-self antigen on its surface

Question 14

Why is the process called cell-mediated response?

Answer 14

The t cells only respond to the antigens that are presented on the antigen presenting cell

Question 15

Describe the process of the cell-mediated response.

Answer 15

1. Once a pathogen has been engulfed by a phagocyte, the antigen is placed on the cell surface and an antigen presenting cell is formed
2. Helper T-cells have receptors on their surface which can attach to the antigens on the APC
3. Once attached, it activates the helper t cells to divide by mitosis in order to replicate and make clones
4. The cloned t helper cells differentiate into different cells.
   - Some of them will remain as t helper cell and activate b lymphocytes
   - Some stimulate macrophages to perform more phagocytosis
   - Some become memory cells for the particular shaped antigen
   - Some become cytotoxic T cells (killer t-cells)

Question 16

What do cytotoxic T cells do and how?

Answer 16

They destroy abnormal or infected cells by releasing a protein called perforin which embeds inside the cell surface membrane and makes a pore allowing any substances to enter or leave which causes cell death.

Question 17

What is a monoclonal antibody?

Answer 17

An antibody produced from identical B cells

Question 18

Give one example of using monoclonal antibodies in a medical treatment.

Answer 18

Binds to specific antigens

Question 19

Explain direct monoclonal antibody therapy.

Answer 19

• Some cancer is treated using monoclonal antibodies that have binding sites complementary in shape to the antigens on the cancer cell.
• The antibodies are given to the cancer patient and they bind onto the antigens on the cancer which prevents the cancer cell from growing.

Question 20

Explain indirect monoclonal antibody therapy.

Answer 20

• Some cancer can be treated using monoclonal antibodies that have binding sites complementary in shape to the antigens on the cancer cells and have drugs attached to them.
• The cancer drugs are delivered directly to the cancer cells to kill them

Question 21

Explain the humoral response.

Answer 21

1. A B-cell is triggered when it encounters its matching antigen
2. The B-cell engulfs the antigen and digests it and displays antigen fragments bound to MHC molecules.
3. This attracts a matching mature Helper T-cell that will activate the B-cell to go through clonal expansion
4. B-cells then undergo mitosis to form either plasma cells or memory B cells.
   - Plasma cells make antibodies
   - Memory B cells can divide rapidly into plasma cells when re-infected with the same pathogen to make large numbers of antibodies rapidly.

Question 22

Describe the structure of HIV

Answer 22

• The core consists of RNA and reverse transcriptase
• Capsid is the outer protein coat
• The lipid envelope is an extra outer layer made out of membrane taken from the hosts cell membrane
• The attachment proteins are on the exterior of the envelope.

Question 23

What is the function of attachment proteins in HIV?

Answer 23

Enables the virus to attach to the hosts helper t cell

Question 24

How does HIV replicate?

Answer 24

1. HIV is transported around the blood until it attaches to a CD4 protein on the Helper T-cells
2. The HIV protein capsule will fuse with the Helper T-cell membrane which allows RNA and reverse transcriptase from the HIV to enter.
3. Reverse transcriptase converts RNA into a DNA which moves into the Helper T-cells nucleus.
4. The DNA is transcribed into mRNA and the Helper T-cells produce viral proteins from the mRNA to make new viral particles which destroys the Helper T-cells. These viral particles are then assembled and release from the cell.

Question 25

Why is it bad when Helper T-cells are destroyed?

Answer 25

The host is unable to produce an effective immune response to other pathogens and is left vulnerable to other infections and cancer

Question 26

What is the structure of an antibody?

Answer 26

• Consists of 4 polypeptide chains
• Variable region is the area that will change in shape to be complementary in shape to the antigen and is where the antigen would bind. This region is different in different antibodies.
• The constant region
• The heavy region
• The light chain
• Receptor binding site

Question 27

What is the role of the disulfide bridge in forming the quaternary structure
of an antibody?

Answer 27

Joins two (different) polypeptides

Question 28

Explain how HIV affects the production of antibodies when AIDS develops
in a person.

Answer 28

HIV destroys the Helper T-cells which means no antibodies are produced which means no B cells can be activated to undergo mitosis to form plasma cells.

Question 29

Give 3 structures found in all viral particles and their function.

Answer 29

Attachment proteins - bind to receptors on cells
Genetic material - codes for protein
Capsid - protects the genetic material

Question 30

Why are viruses described as acellular and non-living?

Answer 30

They have no nucleus and no membrane bound organelles and can only replicate inside a host cell.

Question 31

Why are antibiotics not effective against viruses?

Answer 31

Viruses reproduce inside the host cell which means the bacteria would damage and kill the host cell.