Beta-Glucan
Is a Key Builder and Protector of Your Immune System. The
ANTIOXIDANTS
are another. Together they Work
SYNERGISTICALLY
to Extend the
QUALITY of YOUR LIFE
and as well as its
LENGTH.
The immune
system is a complex network of organs that contain cells that
recognize foreign invaders in the body and destroys them. It
protects the body against pathogens such as viruses, bacteria,
fungi, and other parasites. The immune system offers two kinds of
immunity. One is called humoral innate immunity and involves a
variety of substances found in the humors, or body fluids. These
substances interfere with the growth of pathogens and clump them
together so they can be eliminated from the body. The second is
called cellular innate immunity, which is accomplished by cells
called phagocytes that ingest or eat the pathogens and by natural
killer cells that destroy certain types of cancer cells. Innate
immunity is nonspecific, meaning it is not directed against any
particular invaders, but against any pathogens that attack the body.
An additional
and more sophisticated system of defense is called adaptive immunity
that has the ability to recognize and destroy specific pathogens.
This type of defensive reaction is called the immune response. Any
substance that is capable of generating this type of immune response
is called an antigen or immunogen. Antigens are toxins or enzymes
that the immune systems considers foreign. Immune responses directed
against antigens are called antigen specific. Specificity is one of
the properties that distinguish adaptive immunity from innate
immunity. Adaptive immunity works with innate immunity to provide
the body with a heightened resistance to parasites and other
intruders. Adaptive immunity is also responsible for allergic
reactions and for rejecting transplanted tissue, which it mistakes
for a foreign invader.
Lymphocytes
are a class of white blood cells that are the principal components of
the adaptive immune system. The other components are
antigen-presenting cells, which trap antigens and bring them to the
attention of the lymphocytes. Lymphocytes are different from other
cells in the body because they have nearly 100,000 identical
receptors on their cellular membrane that enables them to recognize
one specific antigen. The receptors are proteins containing grooves
that fit into patterns formed by the atoms of the antigen molecule –
so that the lymphocyte can bind to the antigen. There are more than
ten million different types of grooves in the lymphocytes of the
body’s immune system.
When antigens
invade the body, daughter cells are generated by lymphocytes that have
receptors identical to those found on the original lymphocytes. The
result is a family of lymphocytes, called a lymphocyte clone. The
clone continues to grow after lymphocytes first encounter the
antigen so that if the same type of antigen invades or attacks the
body a second time, there will be plenty of lymphocytes to meet it.
Like all blood cells,
lymphocytes are made from stem cells in the bone marrow. Lymphocytes
then undergo a second stage of development in which they acquire
their antigen-specific receptors. Some lymphocytes are created with
receptors that happen to be specific to normal, healthy components
in the body. A healthy immune system then purges these lymphocytes
and leaves only lymphocytes that ignore normal body components and
only react to foreign pathogens. If the purging process goes amiss,
or is not completely successful, the result is an autoimmune disease
that is in effect, the immune system attacking healthy cells,
molecules or tissue.
Some lymphocytes
are processed in the bone marrow and then migrate to other areas of
the body, specifically to the lymphoid organs. These lymphocytes are
called B cells. Other lymphocytes move from the bone marrow and are
processed in the thymus, a pyramid-shaped organ located beneath the
breastbone. These lymphocytes are called T lymphocytes or T cells
(thymus cells).
These two types of
lymphocytes,
cells and T cells play different roles in the immune response,
although they may act together. The part of the immune response that
involves B cells is often called humoral immunity because it takes
place in the body fluids. The part involving T cells is called
cellular immunity because it takes place directly between the T
cells and the antigens. All adaptive immune responses are cellular,
or initiated by cells (lymphocytes) reacting to antigens. B cells
may initiate an immune response, but the triggering antigens are
actually eliminated by soluble products that the B cells release
into the blood and other body fluids. These products are called
antibodies and belong to a special group of blood proteins called
immun-globulins. When an antigen is encountered in the body fluids
it stimulates a B cell, it transforms, with the aid of a type of T
cell called a helper T cell, into a larger cell called a blast cell.
The blast cell begins to divide rapidly, forming a clone of
identical cells.
Some of these
transform further into plasma cells, antibody-producing factones.
These plasma cells produce a single type of antigen-specific
antibody at a rate of about 2,000 antibodies per second. The
antibodies then circulate through the body fluids, attacking the
triggering antigen.
Antibodies attack
antigens by binding to them. Some antibodies attach themselves to
invading pathogens and render them immobile or prevent them from
penetrating body cells. In other cases, the antibodies act together
with a group of blood proteins, called the complement system, which
consists of at least 30 different components. In these cases,
antibodies coat the antigen and make it subject to a chemical chain
reaction with the complement proteins. The complement reaction
either can cause the invader to burst or it can attract scavenger
cells that ingest or eat the attacker.
Not all the cells
from the clone formed from the original B cell transform into
antibody-producing plasma cells; some serve as memory cells. These
resemble the original B cell, but can respond more quickly to a
second invasion by the same antigen than the original cell.
There are two major
classes of T cells produced in the thymus, helper T cells and
cytotoxic, or killer T cells. Helper T cells secrete molecules
called interleukins that promote the growth B and T cells. The
interleukins that are secreted by lymphocytes are also called
lymphokines. The interleukins that are secreted by other kinds of
blood cells called monocytes and macrophages called monokines.
(IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-5, IL-7, interferon,
lymphotoxin, and tumor necrosis factor). Each interleukin has
complex biological effects.
Cytoxic T cells
destroy cells infected with viruses and other pathogens and may also
destroy cancer cells. Cytoxic T cells are also called suppressor
lymphocytes because they regulate immune responses by suppressing
the function of helper cells so that the immune system is active
only when necessary.
The receptors
of T cells are different from those of B cells because they are
‘trained’ to recognize fragments of antigens that have been combined
with a set of molecules found on the surfaces of all the body’s
cells. These molecules are called MHC, for major histocompatibility
complex. As T cells circulate through the body, they scan the
surfaces of body cells for the presence of foreign antigens that
have been picked up by the MHC molecules. This function is sometimes
called immune surveillance.
When an antigen
enters the body, it may be partly neutralized by components of the
innate immune system. It may be attacked by phagocytes or by
pre-formed antibodies that act together with the complement system.
Often the lymphocytes of the adaptive immune system are also brought
into play.
The human
immune
system contains approximately one trillion T cells and one trillion
B cells, located in the lymphoid organs and in the blood, plus
approximately ten billion antigen-presenting cells located in the
lymphoid organs. To maximize the chances of encountering antigens
wherever they may invade the body, lymphocytes continually circulate
between the blood and certain lymphoid tissues. A given lymphocyte
spends an average of thirty minutes per day in the blood and
re-circulates about fifty times per day between the blood and
lymphoid tissues.
If
lymphocytes
encounter an antigen trapped by the antigen-presenting cells of the
lymphoid organs, lymphocytes with receptors specific to that antigen
stop their migration and settle to mount an immune response locally.
As these lymphocytes accumulate in the affected lymphoid tissue it
often becomes enlarged or swollen.
Antigen-presenting
cells degrade antigens and often eliminate them without the help of
lymphocytes. However, if there are too may antigens for them to
handle alone, the antigen-presenting cells secrete IL-1 and display
fragments of the antigens (combined with MHC molecules) to alert the
helper T cells. The IL-1 facilitates the responsiveness of T and B
cells to antigens and, if released in large amounts, as in the case
of infection, can also drowsiness and fever. Helper T cells that
encounter IL-1 and fragments of antigens transform into cells called
lymphoblasts, which then secrete a variety of interleukins that are
essential to the success of the immune response. The IL-2 produced
by helper T cells promotes the growth of cytotoxic T cells, which
may be necessary to destroy tumor cells or cells infected with
viruses. The IL-3 increases the production of blood cells in the
bone marrow and thus helps maintain an adequate supply of the
lymphocytes and lymphocyte products necessary to fight infections.
Helper T cells also secrete interleukins that act on B cells,
stimulating them to divide to transform into antibody-secreting
plasma cells. The antibodies then perform their part of the immune
function.
The process
of inducing an immune response is called immunization. It may be
either natural through infection of a pathogen, or artificial,
though the use of a serum or vaccine. The heightened resistance
acquired when the body responds to infection is called active
immunity. Passive immunity results when the antibodies from an
actively immunized individual are transferred to a second, nonimmune
subject. Active immunization, whether natural or artificial, is
longer-lasting than is passive immunization because it takes
advantage of immunologic memory.
Scientists
can now produce antibody-secreting cells in the laboratory by a method
known as hybridoma technique. Hybridomas are hybrid cells made by
fusing a cancerous, or rapidly reproducing, plasma cell and a normal
plasma cell obtained from an animal immunized with a particular
antigen. The hybridoma cell can produce large amounts of identical
antibodies, called monoclonal, or hybridoma, antibodies, which have
widespread in medicine and biology.
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