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Biology Unit 1…read more

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Defence mechanisms
Disease is the interaction of body's Defence
Mechanism with the pathogen. There are 2 main
types of defence mechanism.
Non specific: don't distinguish between pathogens
but respond to all in the same way. These act
immediately by either; barrier to entry(skin,
mucus, HCL in stomach) or phagocytosis.
Specific: are slower to act and distinguish between
individual pathogen. They offer long term
immunity and involve a type of white blood cell
called a lymphocyte. They take 2 forms; cell
mediated response by T-lymphocytes or Humoral
response by B-lymphocytes.…read more

Slide 3

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Defence mechanisms
Lymphocytes can distinguish between self and
non-self cells as if not they would attack body'
s own tissues, auto-immune disease. There are
over 10 million lymphocytes present in the
body, which means there is so much variation
that 1 lymphocyte will have a surface protein
that is complementary to an antigen of a
pathogen. The lymphocyte that is
complimentary is stimulated to increase in
numbers. The reason people get ill is because
there is a time delay between getting the
infection and the body treating it.…read more

Slide 4

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The white blood cells phagocytes carry out
this process in the following order:
Chemical products of pathogen act as
attractants, causing phagocytes to move
towards the pathogen.
Phagocytes attach themselves to surface
of pathogen.
They engulf pathogen to form a vesicle,
Lysosomes move towards vesicle and fuse
with it
Enzymes within lysosomes break down
pathogen by hydrolysis.
Soluble products from breakdown of
pathogen are absorbed into cytoplasm of
Histamine dilates the blood vessels which
speeds up delivery of phagocytes to site of
infection. This causes inflammation of
site of infection, which contain dead
pathogens and phagocytes, pus.…read more

Slide 5

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B lymphocytes(B cells) ­ are associated with
Humoral immunity i.e. immunity involving
antibodies that are present in body fluids. They
mature in the Bone marrow.
T lymphocytes (T cells) ­ are associated with
cell-mediated immunity, i.e. immunity
involving body cells. They mature in the
Thymus gland.…read more

Slide 6

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T cells and cell mediated
They respond to organisms own cells that have been invaded
by non self material (viruses or cancer cell). As T cells will
only respond to antigens that are attached to body cell
(rather tan to antigens within body fluid) this type of
response is called cell mediated immunity.
1. Pathogens invade body cells or are taken in by
2. Phagocyte or cell invaded by pathogen places
antigens from pathogen on cell-surface
membrane. These cells are called antigen-
presenting cells.
3. Receptors on certain T helper cells fit exactly
these antigens.
4. This activates other T cells to divide rapidly by
5. Cloned T cells:
Develop into memory cells
Stimulate phagocytes
Stimulate B cells to divide
Kill infected Cells by producing a hole in the
cell surface membrane which makes the cell
freely permeable. This releases viruses which
need living cells in order to reproduce.…read more

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