15/12/2014
Biology- Immunity
Any disease is an interaction between the pathogen and the defence mechanisms of the body.
Sometimes the pathogen overwhelms the body’s defences and the person dies. Sometimes the
body’s defence mechanisms are effective in destroying the pathogen and the individual recovers
from the disease. Having overwhelmed the pathogens in the past, the body’s defences seem to be
better prepared if the same pathogen infects the body again and can repel it before it can cause any
harm to the person. This is known as immunity and is the main reason why certain people are
unaffected by certain pathogens.
There is a large range of the stages of infection and much depends on the general health of the
individual. A fit, healthy adult will rarely fall ill due to an infection, however, those already unwell or
the elderly or very young are much more vulnerable to infection.
Defence mechanisms
The human body has a range of ways it defends itself from pathogens. There are two main types:
Non specific defence mechanisms- They do not distinguish between one pathogen and another, but
respond to all of them in the same way. These take two forms:
a) A barrier to the entry of the pathogen
b) Phagocytosis
Specific defence mechanisms- These mechanisms treat each pathogen differently. The response is
slower but it does provide long lasting immunity. This type of response involves a type of white
blood cell called a lymphocyte and again, their responses take two forms:
a) Cell mediated responses, involving T lymphocytes
b) Humoral responses involving B lymphocytes
In order to be able to defend against pathogens successfully, the body must be able to distinguish
the body’s own cells from foreign material.
Recognising your own cells
To defend the body from invasion from foreign material, lymphocytes must be able to distinguish
the body’s own cells and chemicals (self) from those that are foreign (non self). If they were not able
to do this the lymphocytes would begin to destroy the body’s own tissues.
It is important to remember that specific lymphocytes are not produced in response to an infection,
they already exist, all 10 million different types. Given that there are so many types of lymphocytes,
there is a high probability that, when a pathogen gets into the body, one of the lymphocytes will
have a protein on its surface that is complementary to one of the proteins on the pathogen. In other
words, the lymphocyte can ‘recognise’ the pathogen it works on. Because there are so many types of
lymphocyte, there are very few of each type. When an infection occurs, the complementary
lymphocyte is stimulated to increase its numbers to a level that will be effective in destroying the
infection. This explains the delay between exposure to the pathogen and the body’s defences
reacting.
~1~
, 15/12/2014
How lymphocytes recognise their own cells
There are probably around 10 million different lymphocytes; each one is capable of
recognising a different chemical shape.
In a foetus, these lymphocytes are constantly colliding with other cells
Infection in the foetus is rare because it is protected from the outside world by its mother,
more specifically, the placenta.
Lymphocytes will therefore collude almost exclusively with the body’s own material (self)
Some of these lymphocytes will have receptors that exactly fit those of the body’s own cells.
These lymphocytes are either suppressed, or they die.
The lymphocytes that remain are those that fit foreign material (non-self) and therefore only
respond to foreign material.
Phagocytosis
If a pathogen is to infect the body, it must first gain entry. Clearly, the body’s first line of defence is
to form a physical or chemical barrier to stop the pathogen getting in. If this fails, the next line of
defence is the white blood cells. There are two types of white blood cell: phagocytes and
lymphocytes. Phagocytes ingest and destroy the pathogen (by Phagocytosis) before it can cause
harm. Lymphocytes are involved in immunity.
Barriers to entry
Barriers preventing the entry of pathogens take a number of forms in humans, including:
A protective covering - The skin covers the body surface, providing a physical barrier that
most pathogens cannot penetrate.
Epithelia covered in mucus – Many epithelial layers produce mucus, which acts as a further
defence against invasion. In the lungs, pathogens stick to this mucus, which is then
transported away by cilia, up to the trachea, to be swallowed into the stomach.
Hydrochloric acid in the stomach – This provides such a low pH that the enzymes of most
pathogens are denatured, and therefore the organisms are killed.
Despite these various barriers, pathogens still frequently gain entry and the next line of defence is
then Phagocytosis.
Phagocytosis
Large particles, such as bacteria are far too big to cross cell-surface membranes by diffusion or active
transport. Instead they have to be engulfed by cells in the form of vesicles formed from the cell
surface membrane. This process is called Phagocytosis. In the blood, the types of white blood cell
that perform Phagocytosis are known as phagocytes. They provide an important defence against the
pathogens that manage to enter the body. Some phagocytes travel in the blood can move out of
blood vessels into other tissues.
~2~
Biology- Immunity
Any disease is an interaction between the pathogen and the defence mechanisms of the body.
Sometimes the pathogen overwhelms the body’s defences and the person dies. Sometimes the
body’s defence mechanisms are effective in destroying the pathogen and the individual recovers
from the disease. Having overwhelmed the pathogens in the past, the body’s defences seem to be
better prepared if the same pathogen infects the body again and can repel it before it can cause any
harm to the person. This is known as immunity and is the main reason why certain people are
unaffected by certain pathogens.
There is a large range of the stages of infection and much depends on the general health of the
individual. A fit, healthy adult will rarely fall ill due to an infection, however, those already unwell or
the elderly or very young are much more vulnerable to infection.
Defence mechanisms
The human body has a range of ways it defends itself from pathogens. There are two main types:
Non specific defence mechanisms- They do not distinguish between one pathogen and another, but
respond to all of them in the same way. These take two forms:
a) A barrier to the entry of the pathogen
b) Phagocytosis
Specific defence mechanisms- These mechanisms treat each pathogen differently. The response is
slower but it does provide long lasting immunity. This type of response involves a type of white
blood cell called a lymphocyte and again, their responses take two forms:
a) Cell mediated responses, involving T lymphocytes
b) Humoral responses involving B lymphocytes
In order to be able to defend against pathogens successfully, the body must be able to distinguish
the body’s own cells from foreign material.
Recognising your own cells
To defend the body from invasion from foreign material, lymphocytes must be able to distinguish
the body’s own cells and chemicals (self) from those that are foreign (non self). If they were not able
to do this the lymphocytes would begin to destroy the body’s own tissues.
It is important to remember that specific lymphocytes are not produced in response to an infection,
they already exist, all 10 million different types. Given that there are so many types of lymphocytes,
there is a high probability that, when a pathogen gets into the body, one of the lymphocytes will
have a protein on its surface that is complementary to one of the proteins on the pathogen. In other
words, the lymphocyte can ‘recognise’ the pathogen it works on. Because there are so many types of
lymphocyte, there are very few of each type. When an infection occurs, the complementary
lymphocyte is stimulated to increase its numbers to a level that will be effective in destroying the
infection. This explains the delay between exposure to the pathogen and the body’s defences
reacting.
~1~
, 15/12/2014
How lymphocytes recognise their own cells
There are probably around 10 million different lymphocytes; each one is capable of
recognising a different chemical shape.
In a foetus, these lymphocytes are constantly colliding with other cells
Infection in the foetus is rare because it is protected from the outside world by its mother,
more specifically, the placenta.
Lymphocytes will therefore collude almost exclusively with the body’s own material (self)
Some of these lymphocytes will have receptors that exactly fit those of the body’s own cells.
These lymphocytes are either suppressed, or they die.
The lymphocytes that remain are those that fit foreign material (non-self) and therefore only
respond to foreign material.
Phagocytosis
If a pathogen is to infect the body, it must first gain entry. Clearly, the body’s first line of defence is
to form a physical or chemical barrier to stop the pathogen getting in. If this fails, the next line of
defence is the white blood cells. There are two types of white blood cell: phagocytes and
lymphocytes. Phagocytes ingest and destroy the pathogen (by Phagocytosis) before it can cause
harm. Lymphocytes are involved in immunity.
Barriers to entry
Barriers preventing the entry of pathogens take a number of forms in humans, including:
A protective covering - The skin covers the body surface, providing a physical barrier that
most pathogens cannot penetrate.
Epithelia covered in mucus – Many epithelial layers produce mucus, which acts as a further
defence against invasion. In the lungs, pathogens stick to this mucus, which is then
transported away by cilia, up to the trachea, to be swallowed into the stomach.
Hydrochloric acid in the stomach – This provides such a low pH that the enzymes of most
pathogens are denatured, and therefore the organisms are killed.
Despite these various barriers, pathogens still frequently gain entry and the next line of defence is
then Phagocytosis.
Phagocytosis
Large particles, such as bacteria are far too big to cross cell-surface membranes by diffusion or active
transport. Instead they have to be engulfed by cells in the form of vesicles formed from the cell
surface membrane. This process is called Phagocytosis. In the blood, the types of white blood cell
that perform Phagocytosis are known as phagocytes. They provide an important defence against the
pathogens that manage to enter the body. Some phagocytes travel in the blood can move out of
blood vessels into other tissues.
~2~
