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Cells (AQA AS Biology) PART 6 of 6 TOPICS
Cell recognition and the immune system:
Each cell in our body has an antigen which tells the body whether it is foreign (not part of the body) or
self (is part of the body). An antigen is a protein on the surface that triggers an immune response.
Certain influenza viruses keep changing their antigens to avoid an attack by antibodies and T cells.
Phagocytosis should be known: Receptors on the phagocyte (a white blood cell) attach to the antigens
on the microorganism which is called opsonisation (NB: Opsonisation does not need to be known).
The microorganism is then engulfed into a vesicle called a phagosome. A lysosome fuses all its
hydrolytic enzymes such as lysozymes into the phagosome to make it a phagolysosome. Once the
microorganism has been hydrolysed it will be eliminated by exocytosis where it leaves via a vesicle.
(NB: You have to say hydrolysed and not broken down as you will not get a mark). The antigens that
were on the antigen are used by the phagocyte on the surface to become an antigen-presenting cell.
This phagocyte interacts with a T-Helper cell that can recognise the complementary antigen. Once it
has binded it causes the number of cytotoxic T cells and B cells to increase. Normal body cells can
become antigen-presenting cell when infected. Complementary cytotoxic T cells bind to the antigen to
produce chemicals which kill the infected cell. With B cells, if a T-Helper cell binds to a B cell it will
cause it to differentiate into many plasma cells that make the same antibodies but in bigger numbers.
One the pathogens have been eliminated most of the plasma cells are destructed with a few plasma
cells becoming memory cells so that a secondary response is faster and better when a second invasion
happens. These memory cells last for several decades.
An antigen is a protein that forms antigen-antibody complexes when complementary paired with an
Agglutination is where antibodies called agglutinins link bacterial cells together to form a clump called
an agglutinate. Some viruses react to the surface of red blood cells (the haemaglutinins) to cause
Herd immunity is where a significant amount of people in a population are vaccinated to stop the
spread of infections. If only a few people are vaccinated then herd immunity will fail.
Active immunity is where the body makes its own antibodies against pathogens whereas passive
immunity is where antibodies are made by someone else.
Replication of HIV is needed to be known: HIV has glycoproteins as its receptors on a lipid envelope. It
has a capsid in the middle which contains an RNA molecule and reverse transcriptase, integrase and
protease. It attaches its receptors onto a T-Helper cell by complementary shapes. The HIV molecule
enters the cell by endocytosis leaving all its glycoproteins on the surface of the cell and fusing its RNA
molecule and three enzymes. Reverse transcriptase makes a DNA molecule using the RNA strand and
degrades the RNA strand. A second DNA molecule is made and becomes a circular DNA molecule. This
is then incorporated into the host's DNA by the enzyme integrase. This has now become proviral DNA
and this can remain dormant. RNA polymerase uses this to make mRNA and the viral genome RNA.
The mRNA is translated to make viral enzymes and viral protein structures. Protease breaks some of
the poly-proteins up which is used as a core round the viral RNA and is released by budding out of the
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This is continued to be made without the
damaging the body until the cell is ruptured. The same HIV molecules can target many more T-Helper
cells leading to AIDS when he body's immune system has weakened.
Some monoclonal antibodies deliver chemotherapy direct to the cancer cells which then are
Pregnancy test kits have monoclonal antibodies which bind to a hormone called HCG which is passed
in the urine.…read more