AQA AS Biology Unit 1: Immunity

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Any disease is an interaction between the pathogen and the body's various defence mechanisms.
Sometimes the pathogen overwhelms the defences and the individual dies. Sometimes the body's
defence mechanisms overwhelm the pathogen and the individual recovers from the disease.
Having overwhelmed the pathogen, however, the body's defences seem to be better prepared for a
second infection from the same pathogen and can repel it before it can cause any harm. This is
known as immunity and is the main reason why some people are unaffected by certain pathogens.
Immunity: the ability of an organism to resist disease. It involves the recognition of foreign
material and the production of chemicals which help to destroy it.
The Immune System
The immune system is the body's defence system against disease. It consists mainly of the white
blood cells, but parts of the immune system are spread all over the body.
They include:
- The lymph and blood vessels: contain transport pathogens and leukocytes all over the body
- The lymph nodes: contain millions of phagocyte and lymphocyte cells which identify and
remove pathogens from lymph
- The spleen: contain millions of phagocyte and lymphocyte cells which identify and
remove pathogens from lymph
- The thymus: where blood stem cells are differentiated into T-lymphocytes

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The human body has a range of defences to protect itself from pathogens.
They are of two main types:
Non-Specific Mechanisms that do not distinguish between one type of pathogen and another,
but respond to all of them in the same way.
These mechanisms act immediately and take two forms:
- a barrier to the entry of pathogens
- phagocytosis
Specific Mechanisms that do distinguish between different pathogens.
The responses are less rapid but provide long-lasting immunity.…read more

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If a pathogen is to infect the body it must first gain entry.
Clearly then, the body's first line of defence is to form a physical or chemical barrier to entry.
Should this fail, 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 a process called phagocytosis before it can
cause harm.…read more

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Phagocyte is attracted to the pathogen by chemoattractants
2. It moves towards the pathogen along a concentration gradient
3. The phagocyte binds to the pathogen
4. Lysosomes within the phagocyte migrate towards the phagosome
formed by engulfing the bacterium
5. The lysosomes release their lytic enzymes into the phagosome where
they break down the bacteria
6.…read more

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An antigen is any part of an organism or substance that is recognised as non-self or foreign by
the immune system and stimulates an immune response.
Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading
cells, such as microorganisms, or diseased cells, such as cancer cells.
The presence of an antigen triggers the production of an antibody as part of the body's defence
Immune responses such as phagocytosis are non-specific and occur whatever the infection.…read more

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Cell Meditated Immunity
T lymphocytes respond to an organism's own cells that have been invaded by non-self
material, e.g. a virus or a cancer cell. They also respond to transplanted material, which is
generally different.…read more

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B Cells and Humoral Immunity
Humoral immunity is so called because it involves antibodies and antibodies are soluble in the
blood and tissue fluid o the body.
Another word for body fluids is humour.
There are many different types of B cells and each type produces a different antibody that
responds to one specific antigen.
When an antigen e.g.…read more

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The surface antigens of the invading pathogen are taken up by B cells
2. The B cells process the antigens and present them on their surfaces
3. T helper cells attach to the processed antigens on the B cells thereby activating them
4. The B cells are now activated to divide by mitosis to give a cline of plasma cells
5. The cloned plasma cells produce antibodies that exactly fit the antigens on the pathogens
6.…read more

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Antibodies are proteins synthesised by B cells. When the body is invaded by non self material, a B
cells produces antibodies. These antibodies react with antigens on the surface of non-self
material by binding to them precisely.
Antibodies are therefore very specific, each antigen having its own individual antibody. This
massive variety of antibodies is possible because they are made of proteins ­ molecules that occur in
almost an infinite number of forms.
Antibodies are made up of four polypeptide chains.…read more

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Monoclonal Antibodies
Bacterium or other microorganisms that enter the body are likely to have hundreds of different
antigens on its surface. Each antigen will induce a different B cell to multiply and clone itself.
Each of these clones will produce a different antibody, known as polyclonal antibodies.
In the 1970s researchers wanted to be able to obtain large amounts of one particular antibody at a
time.…read more


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