Biology - Cell recognition and the immune system

Question 1

Pathogen

Answer 1

Any microorganism that causes disease

Question 2

Non-specific defence mechanism

Answer 2

Response is immediate and the same for all pathogens
Physical barrier
Phagocytosis

Question 3

Specific defence mechanism

Answer 3

Response is slower and specific to each pathogen
Cell-mediated response (T-lymphocytes)
Humoral response (B-lymphocytes)

Question 4

How do lymphocytes recognise cells belonging the body?

Answer 4

• ten million lymphocytes which recognise different shapes
• In fetus the lymphocytes are colliding mostly with body’s own material. Infection in fetus is rare due to being protected by the placenta
• Some lymphocytes will have receptors on the surface that fit directly into body cells. They either die or are suppressed. The remaining only respond to foreign material (non-self)

Question 5

Examples of white blood cells

Answer 5

Lymphocytes and phagocytes

Question 6

Phagocytosis

Answer 6

Large particles, such as some types of bacteria, can be engulfed by cells in the vesicles formed from the cell-surface membrane.

Question 7

Process of phagocytosis (step by step)

Answer 7

1. The phagocyte is attracted to the pathogen by chemical products of the pathogen. It moves towards the pathogen along a concentration gradient.
2. The phagocyte has several receptors on its cell-surface membrane that attach to chemicals on the surface of the pathogen. The phagocyte engulfs the pathogen to form a vesicle called a phagosome.
3. Lysosomes within the phagocyte migrate towards the phagosome.
4. The lysosomes release their lysozymes into the phagosome, where they hydrolyse the bacterium
5. The hydrolysis products of the bacterium are absorbed by the phagocyte.

Question 8

Antigen

Answer 8

Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.
Usually proteins that are on the cell-surface membrane of invading cells.

Question 9

B-lymphocytes (B cells) role

Answer 9

Mature in the bone marrow and associated with humoral immunity (antibiotics that are present in the bodily fluids or blood plasma).

Question 10

T-lymphocytes (T cells) role

Answer 10

Mature in the thymus gland and are associated with cell-mediated immunity (involving body cells)

Question 11

Why can T-lymphocytes distinguish normal cells from invader cells?

Answer 11

1) Phagocytes that have engulfed and hydrolyzed a pathogen present some of the pathogen’s antigens on its surface.
2) Body cells invaded by a virus present some of the viral antigens on their cell surface membrane
3) Transplanted cells from individuals of the same species have different antigens on their cell- surface membrane
4) Cancer cells are different from body cells and present antigens on their cell-surface membrane.

Question 12

What are cells that display foreign antigens on their surface membrane called?

Answer 12

Antigen-presenting cells

Question 13

Steps of cell-mediated immunity (T-lymphocytes)

Answer 13

1. Pathogens invade body cells
2. Phagocyte places antigens from the pathogen on cell-surface membrane
3. Receptors on a specific helper T cell fit exactly into those antigens
4. This activates T cells to divide rapidly by mitosis and form genetically identical clones: they can develop into memory cells, stimulate B cells to divide and secrete their antibody and activate cytotoxic T cells.

Question 14

How do Cytotoxic T cells kill infected cells

Answer 14

They kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin that makes holes in the surface membrane. This allows it to become freely permeable to all substances and cells die as a result. This highlights the importance of cell surface membranes in maintaining the integrity of the cell (action of T cells is most effective).

Question 15

How are B cells specific?

Answer 15

There are many different types of B cells each specific to produce a specific antibody that responds to one specific antigen. There is a complementary fit.

Question 16

What is clonal selection?

Answer 16

Helper T cells bind to these processed antigens and stimulate B cells to divide by mitosis. This will create identical clones of B cells that produce antibodies specific to the foreign antigen.

Question 17

What is a monoclonal antibody?

Answer 17

Each clone produces one specific antibody

Question 18

In each clone, the cells develop into one of two types of cells: What is the role of plasma cells?

Answer 18

They secrete antibodies into the blood plasma. They only survive for a few days but a vast amount is made. They are responsible for the immediate defense of the body against infection. Primary immune response leads to the destruction of the antigen.

Question 19

What is the role of memory cells?

Answer 19

Responsible for the secondary immune response. They live longer but do not produce antibodies directly, they circulate in the blood and tissue fluid. They divide rapidly and develop into more memory cells as well as plasma cells. The plasma cells further destroy antigens and memory cells circulate in readiness for further infection. It ensures that the infection is destroyed before any harm is caused.

Question 20

Summary of role of B cells in humoral immunity

Answer 20

1. Surface antigens of invading pathogens is taken up by B cells
2. The B cells process the antigens and display them on its surface membrane
3. Helper T cells are activated and attach to the antigens which will activate the B Cells.
4. The B cells divide my mitosis to create cloned plasma cells which produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface.
5. The antibody attaches to antigens on the pathogen and destroy them
6. Some B cells develop into memory cells and can respond to any future infection by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response.