5.1 Defence mechanisms

Question 1

What is an infection

Answer 1

An interaction between a pathogen and the body’s various defence mechanisms

If the pathogen overwhelms the body’s defences, the individual may die, but if the body’s defence mechanisms overwhelm the pathogen, the individual recovers from the disease

Question 2

What is immunity

Answer 2

Having overwhelmed the pathogen, the body’s defences are better prepared for a second infection of the same pathogen so they can kill it before it causes any harm

Question 3

How can our body’s recognise our own cells in the fetus

Answer 3

. There are around 10 million different lymphocytes present at any time, and each can recognise a different chemical shape

. In the fetus these lymphocytes collide with other cells that are almost always our own material as infection in the fetus is rare as its protected from the outside world by the mother and PLACENTA

. Some lymphocytes have receptors that exactly fit the body’s own cells, so these die or are suppressed.

. The only remaining lymphocytes are those that might fit foreign material so only respond to foreign material

Question 4

How can our lymphocytes recognise cells belonging to the body in adults

Answer 4

. Lymphocytes produced in bone marrow initially only encounter self-antigens

. So any lymphocytes that show an immune response to these self antigens undergo programmed cell death (APOPTOSIS) before they can differentiate into mature lymphocytes

. No clones of these anti-self
lymphocytes will appear in the blood, leaving only those who respond to non-self antigens

Question 5

What is the role of lymphocytes

Answer 5

. Lymphocytes must be able to distinguish between foreign cells, and our body cells to defend the body from invasion, as if they couldn’t do this they would destroy the organisms tissues

Question 6

How can cells be identified

Answer 6

. Every type of cell (self or non self) has specific molecules on its surface that identify it, eg proteins that have an enormous variety and a highly specific tertiary structure that allows them to be distinguishable

Question 7

What do the protein molecules on cell surfaces allow the immune system to identify

Answer 7

. Pathogens

. Non self material such as cells from other organisms of the same species

. Toxins including those produced by certain pathogens

. Abnormal body cells eg cancer

Question 8

What are non specific defence mechanisms and give two examples

Answer 8

. Response is immediate and the same for all pathogens
Eg phagocytosis
Eg Physical barriers like skin

Question 9

What are specific defence mechanisms and give two examples

Answer 9

Response is slower and specific to each pathogen
Eg cell-mediated response such as
T lymphocytes
Eg humoral response such as
B lymphocytes

Question 10

Why does the immune response pose problems for those getting organ transplants

Answer 10

. The immune system recognises them as non self even if they come from individuals of the same species so the body destroys the transplant

. To minimise the effect of this tissue rejection, the transplants have to be a match as closely as possible eg from relatives

. Immunosuppressant drugs are also taken

Question 11

Specific lymphocytes aren’t produced in response to an infection, they already exist in the body so how does this work

Answer 11

There are 10 million types of lymphocyte so there is a high probability that when a pathogen enters the body, there will be a lymphocyte that has the correct protein on its surface that is complementary to it .

When an infection occurs, the lymphocyte that is complementary can be stimulated to divide and build up its numbers to it can destroy it.

This is called clonal selection