14.1

Question 1

What is the general function of the body’s defence mechanisms? (1)

Answer 1

To protect the body from pathogens

Question 2

What are pathogens? (1)

Answer 2

disease-causing microorganisms

Question 3

Describe the 2 broad categories of defence mechanisms in the body. (4)

Answer 3

(1) non-specific —> (3) immediate, same response for all pathogens
(2) specific —> (4) slower, specific to each pathogen

Question 4

Give an example of a non-specific physical barrier defence. (1)

Answer 4

skin

Question 5

What type of white blood cell is involved in non-specific, longer-lasting defence? (1)
What is the process called? (1)

Answer 5

(1) Phagocytes
(1) phagocytosis

Question 6

What type of white blood cells are involved in specific defence mechanisms? (1)

Name the two types of responses they mediate & mention what specific type of white blood cells they involve. (4)

Answer 6

(1) Lymphocytes

(1) cell-mediated responses (3) (involving T lymphocytes)
(2) humoral responses (4) (involving B lymphocytes)

Question 7

What is the scientific name for “white blood cells”? (1)

Answer 7

Leucocytes

Question 8

What is immunity? (1)

Answer 8

the ability of the body to be prepared for a second infection by the same pathogen and destroy it before it causes harm

Question 9

What is the fundamental requirement for lymphocytes to defend the body from foreign material? (1)

Answer 9

The ability to distinguish between the body’s own cells and molecules (self) from those that are foreign (non-self)

Question 10

What would happen if lymphocytes could not distinguish between self and non-self? (1)

Answer 10

They would destroy the organism’s own tissues

Question 11

What are antigens? (2)

Answer 11

(1) specific molecules on the surface of cells (self or non-self)
(2) that identify them

Question 12

What type of molecule is most important as an antigen and why? (2)

Answer 12

(1) Proteins
(2) because they have enormous variety and a highly specific tertiary (3-D) structure

Question 13

What feature of proteins allows the immune system to distinguish one cell from another? (1)

Answer 13

The variety of their specific 3-D structures

Question 14

Name 5 types of antigen-presenting cells (APCs). (5)

Answer 14

(1) pathogens
(2) donor / transplanted cells OR non-self material (from other organisms of the same species)
(3) toxins (which is produced by certain pathogens)
(4) abnormal body cells e.g. cancer cells
(5) phagocytes

Question 15

Why can tissue or organ transplants lead to rejection? (2)

Answer 15

(1) The recipient’s immune system recognizes the antigens on the donor tissue as non-self
(2) and attempts to destroy them

Question 16

How are donor tissues for transplant typically matched to recipients? (1)

Answer 16

As closely as possible, ideally with those of relatives who are genetically close

Question 17

What type of drugs are often administered to transplant recipients and why? (2)

Answer 17

(1) immunosuppressant drugs
(2) to reduce the level of the immune response and prevent rejection

Question 18

How does the body ensure that lymphocytes do not attack its own cells? (2)

Answer 18

(1) During development in the fetus, lymphocytes constantly collide with the body’s own cells.
(2) Lymphocytes with receptors that fit the body’s own cells either die or are suppressed

Question 19

Why is infection in a fetus rare? (1)

Answer 19

because it’s protected from the outside world by the mother & the placenta

Question 20

What happens to lymphocytes in the fetus that show an immune response to self-antigens? What is this process called? (2)

Answer 20

(1) They undergo programmed cell death (apoptosis)
(2) before they can differentiate into mature lymphocytes

Question 21

How many different types of lymphocytes are thought to be present in the body at any time? (1)
What is the significance of this variety? (1)

Answer 21

(1) around 10 million

(1) this high probability ensures that when a pathogen enters the body, one of these lymphocytes will have a protein on its surface complementary to one of the pathogen’s proteins

Question 22

What happens when a lymphocyte encounters an antigen that is complementary to its surface receptor? (2)

What is this process called? (1)

Answer 22

(1) It is stimulated to divide & build up its numbers
(2) to a level where it can be effective in destroying the pathogen

(1) clonal selection

Question 23

Why is there a time lag between exposure to a pathogen and the body’s defences bringing it under control? (2)

Answer 23

(1) It takes time for the specific lymphocyte with the complementary receptor to the pathogen’s antigens
(2) to be selected and undergo clonal expansion to a sufficient level

Question 24

State two differences between a specific and a non-specific defence mechanism. (2)

Answer 24

Specific:
- distinguishes between different pathogens
- responds more slowly

Non-specific:
- treats all pathogens in the same way
- responds more rapidly

Question 25

After a pathogen gains entry to the body, it is often a number of days before the body's immune system begins to control it. Suggest a possible reason why this is so. (2)

Answer 25

(1) This time lag is due to the process of **clonal selection & expansion** (2) where the **specific lymphocyte** that **recognizes** the **pathogen needs to be identified** and **multiplied to an effective number**

Question 26

Why would it be inaccurate to say that the body takes days to 'respond' to the pathogen? (2)

Answer 26

(1) the body responds **immediately** by **'recognising'** the pathogen (& by phagocytosis) (2) the **delay** is in **building up numbers of lymphocytes** & therefore **controlling the pathogen**