OCR AS Biology- The Immune System

Complete set of revision notes on the immune system for OCR Biology, AS level.

Contains: Key Definitions, Diagrams and Explanations

Topics: Antibodies, Antigens, Disease, Primary and Secondary Response, Lymphocytes, Phagocytes, Macrophages and Pathogens.

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  • Created on: 14-03-13 14:22
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Basic Immunity
Parasite ­ organisms that live in or on another living thing, harming their host,
usually by taking their nutrients. External parasites live on the host, e.g. head lice,
and internal parasites live inside the host, e.g. tapeworm.
Pathogens ­ pathogens are organisms that cause disease, taking nutrients from
the host while also causing damage in the process.
Malaria ­ Plasmodium, a protoctista. Carried by a vector; the female Anopheles
mosquito. The mosquito sucks the blood of an infected person, and the parasite
gametes fuse and form zygotes. They then migrate to the salivary glands of the
mosquito, and when this mosquito bites an uninfected person, a little saliva is
injected as an anticoagulant. The malarial parasite then passes through the blood
to the liver, where they multiply before entering the blood once again
HIV/AIDS ­ Caused by the human immunodeficiency virus. Can remain inactive in
the body for a long period of time, but when it becomes active it attacks and
destroys T helper cells. This weakens the immune system, and a range of
opportunistic infections may strike. AIDS is the latter stages of the disease, and
this stands for acquired immune deficiency syndrome. Transmitted by
unprotected sex, blood to blood contact, unscreened blood transfusions and use
of unsterile surgical equipment.
TB ­ caused by Mycobacterium tuberculosis, transmitted by droplet infection, by
sneezing. Spreads easily if conditions are overcrowded with poor ventilation, poor
health, poor diet and homelessness
Immune response ­ The specific response to a pathogen, which involves the
action of lymphocytes and the production of antibodies

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Antigen ­ molecules which stimulate an immune response. A foreign antigen will
be detected by the immune system and will stimulate the production of antibodies.
These antibodies will be specific to the antigen. As the antigen is specific to the
organism, the antibody is specific to the pathogen. Antigens are usually a protein or
glycoprotein in or on the plasma membrane. Our own antigens recognised by our immune
system do not stimulate any response.…read more

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Mode of action of antibodies:
-The variable region of the antibody binds to a pathogen's antigens, which
lay on the cell membrane. This is known as neutralisation. This antigen
may be used as a binding site, so when the antibody binds to that antigen
the pathogen will be unable to bind to the host cell. Some antibodies
resemble several Y-shaped molecules, and these have many variable
regions. These attach to many pathogens, causing agglutination.…read more

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Macrophages partially break down pathogens, separating the antigens
from the rest of the microorganism. It then displays this antigen on its cell
membrane, becoming an antigen-presenting cell. It then travels round the body
to help with clonal selection, finding the correct T lymphocyte. They also release
monokines that attract neutrophils by chemotaxis. They also release monokines
that can stimulate B cells to differentiate and release antibodies.…read more


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