AS Unit 1 Biology - Everything on IMMUNITY!

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Section 6.1 ­ Defence Mechanisms
A 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 defence mechanisms overwhelm the
pathogen and the individual makes a full recovery. The body's defences are
now better prepared for a second infection from the same pathogen ­ win
This is known as immunity.
Defence Mechanisms ­ Two Main Types!
Recognising Your Own Cells
Lymphocytes must be able to distinguish the body's own cells and chemicals
(self) from those that are foreign (non-self). Otherwise, lymphocytes would
destroy the organisms own tissues.
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Section 6.…read more

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For a pathogen to infect the body it must at first gain entry. The body's first
line of defence is to form a physical or chemical barrier. Should this fail, bring on
the white blood cells (there are two types: phagocytes and lymphocytes).
Lymphocytes are involved in immunity, and phagocytes ingest and destroy the
pathogen by a process called phagocytosis before it can cause any harm.
Barriers to Entry
1) A PROTECTIVE COVERING. Skin, obviously.…read more

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Chemical products of the pathogen act as attractants which draw the
phagocyte towards it.
Pathogens are engulfed by phagocytes in the form of vesicles which are
formed on the cell-surface membrane.
Phagocytes attach themselves to the surface of the pathogen.
They engulf the pathogen to form a vesicle known as a "phagosome".
Enzymes within the Lysosomes join with the phagosome and release
their contents. The enzymes within the Lysosomes digest the pathogen.…read more

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The initial response of the body to infection is non-specific. The next phase is
the specific response that confers immunity, the ability of organisms to resists
infection by protecting against disease-causing microorganisms that invade
their bodies. It involves the recognition of foreign material ­ antigens.
Any part of an organism that is recognised as non-self (foreign) by the
immune system and stimulates an immune response.
Usually proteins that are part of the cell-surface membranes or cell walls
of invading cells.…read more

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T ­ Lymphocytes can distinguish foreign material from the body's own tissue
1) Phagocytes that have engulfed and broken down a pathogen present
some of its antigens on its own cell-surface membrane.
2) Body cells invaded by a virus also manage to present some of the viral
antigens on its surface as a sign of distress.
3) Cancer cells also present antigens on their cell-surface membranes.…read more

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Section 6.…read more

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In section 6.3 we saw the first phase of the specific response to infection is the
cloning of the relevant T cells to build up their numbers. Some of these T cells
produce factors that stimulate B cells to divide, which are involved in the next
stage of the specific response: humoral immunity.
What is Humoral Immunity?
Humoral immunity is so called because it involves antibodies which are soluble
in the blood and tissue fluid of the body, also called "humour".…read more

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T helper cells attach to the processed antigens and B cells thereby
activating them.
4. The B cells are now activated to divide by mitosis to give a clone of the
plasma cells.
5. The cloned plasma cells produce antibodies that exactly fit the antigens
on the pathogens surface.
6. The antibodies attach to antigens on the pathogens and destroy them.
This is the primary immune response.
7. Some B cells develop into memory cells.…read more

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Monoclonal antibodies
A pathogen entering the body is likely to have hundreds of different antigens
on its surface. Each antigen will induce a different B cells to divide and clone its
self. Each clone will produce a different antibody known as a polyclonal
Obviously, it is of considerable medical value to be able to produce antibodies
outside of the body. Antibodies that can be isolated and cloned are called
monoclonal antibodies.…read more


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