3-5.
ACTION OF A VIRUS ON A CELL
A variety of microorganisms are present in our environment. Some of these are
harmful--bacteria, fungi, rickettsiae, protozoans, helminthes, and viruses--and can
cause disease in a susceptible person. AIDS is caused by a virus called the human
immunodeficiency virus (HIV).
a. Viruses. A virus can be defined as a piece of free-floating genetic material
which is surrounded by a protective protein coating called a capsid. Not a cell and not
alive, a virus is a subcellular microorganism whose duty is to replicate (reduplicate,
duplicate).
(1) When a virus enters the body, it finds a host cell. Once the virus selects
a cell, the virus can pierce the cell membrane with a "spike" which is in the capsid of the
virus or the virus can use an enzyme to break down the cell membrane and enter the
cell. Inside the cell, the virus uses the cell's machinery to replicate itself. The cell will
die either from the duplication process or when the new viral particles leave the cell. On
the other hand, sometimes the virus will become dormant (do nothing) when it enters a
cell.
(2) After a virus enters a cell (infection), the body's primary immune
response will go into action. B cells and T cells collect at the site of the infection to fight
the virus. Antibodies to this particular virus are produced by the B cells. T cells have
antibodies in their cell membranes. T cells will determine the type of virus. The
particular T cell which can fight this virus will become active, increasing in size and
multiplying. This killer T cell will rush to the cell with the virus and secrete substances
that render the viral cell harmless. (Thousands of different types of T cells exist in the
body; each type of T cell can fight a specific infection.)
b. Action of HIV on Cells. AIDS is caused by the action of a virus, the human
immunodeficiency virus (HIV), on cells in the body. The problem is that the human
immunodeficiency virus attacks the very cells that are responsible for killing the virus,
the T4 lymphocytes. HIV enters the bloodstream and makes contact with T4
lymphocytes (white blood cells). The invading virus fits onto the cells' receptors. The
cell reads the virus as if the virus were part of the cell. At this point, the virus can either
become dormant (inactive and just being there), or the virus can become active,
changing and multiplying to enter the bloodstream to infect other T4 cells. If this virus is
active, it can multiply itself many times, break out of the host T4 cell, killing the cell in
the replication process or as the virus leaves. What happens in the body is that T4 cells
are either killed or weakened. Since the T4 cells are of great importance in the body's
immune system, eventually after the major portion of these cells is killed, the body's
immune system is weakened or useless. Then the patient becomes susceptible to
opportunistic infections and unusual cancers. The patient has developed the syndrome
called AIDS.
NOTE:
A dormant virus in a cell may become active at any time.
MD0580
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