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Immune System
OCR AS Biology…read more

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Primary Defences
This acts as a physical barrier blocking pathogens
from entering the body. It can also act as a
chemical barrier by producing chemicals that are
antimicrobial and can lower the pH, inhibiting
the growth of pathogens
Mucous membranes
They protect body opening that are exposed to the environment (such as mouth, nostrils, ears) Some
membranes secret mucus ­ a sticky substance that contains antimicrobial enzymes. The epithelial
layer in the lungs contains mucus secreting goblet cells. Most pathogens in the digestive system are
killed by the acidity of the stomach which can be 1-2 pH This denatures the pathogens enzymes.
The eyes are also protected by antibodies in tear fluid.…read more

Slide 3

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Phagocytosis ­ a secondary defence
1) A phagocyte recognises the antigens on a pathogen
2) The cytoplasm of the phagocyte moves around the
pathogen, engulfing it
3) The pathogen is now contained in a phagocytic
vacuole ( a bubble) in the cytoplasm of the phagocyte
4) A Lysosome fuses with the phagocytic vacuole. The
digestive enzymes break down the pathogen
5) The phagocyte then presents the pathogen's
antigens. It sticks the antigens on the surface to
activate other immune system cells.
There are two types of phagocyte;
· Neutrophils; the most common type, are manufactured in the bone marrow. They travel in the
blood and often squeeze out into the tissue fluid. Neutrophils are short lived but are released in
large numbers as a result of infection.
·Macrophages; are larger cells also manufactured in the bone marrow. The travel in blood as
monocytes. They tend to settle in the lymph nodes. Here they develop into macrophages.…read more

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Antigens are molecules that stimulate an immune response
Antibodies are protein molecules that can identify and neutralize antigens
The structure of an Antibody ·Four poly peptide chains held together
by disulphide bridges
·A constant region which is the same in
all cells this allows the antibody to
attach to the phagocytic cells, helping
the process of phagocytises
·A variable region which has a specific
shape. This is a result of its amino acid
sequence. It ensures that the antibody
can attach to the complementary
shaped antigen.
·Hinge regions which allow a degree of
flexibility. These allow the branches of
the Y shaped molecule to move apart
so they can attach to more than one
antigen.…read more

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How antibodies work
A large antibody, which resembles several
Y shaped molecules attached together
with severable variable regions, can bind
many pathogens together. The group of
pathogens is now too large to enter the
host cell.
Most Antibodies cover the pathogen
binding sites to prevent the pathogen
from binding to a host cell and entering
the cell…read more

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Producing Antibodies
Antibodies are produced in response
to an infection. However it takes a
few days for there to be enough
antibodies in the blood to combat
the infection. This is known as the
primary immune response. Once the
pathogens have been dealt with the
number of antibodies drops
If the body is infected by the same
pathogen, the antibodies must be
made again. But the immune system
can swing into action more quickly.
The production of antibodies starts
sooner and is much more rapid. The
concentration of antibodies rises
sooner and reaches a higher
concentration. This is known as
secondary immune response.…read more

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