The overall function of the immune system is to prevent or limit infection.
The immune system can distinguish between normal, healthy cells and unhealthy
cells by recognizing a variety of "danger" cues called danger-associated
molecular patterns (DAMPs). Cells may be unhealthy because of infection or because
of cellular damage caused by non-infectious agents like sunburn or cancer. Infectious
microbes such as viruses and bacteria release another set of signals recognized by
the immune system called pathogen-associated molecular patterns (PAMPs).
When the immune system first recognizes these signals, it responds to address the
problem. If an immune response cannot be activated when there is sufficient need,
problems arise, like infection. On the other hand, when an immune response is
activated without a real threat or is not turned off once the danger passes,
different problems arise, such as allergic reactions and autoimmune disease.
The immune system is complex and pervasive. There are numerous cell types that
either circulate throughout the body or reside in a particular tissue. Each cell
type plays a unique role, with different ways of recognizing problems, communicating
with other cells, and performing their functions. By understanding all the details
behind this network, researchers may optimize immune responses to confront specific
issues, ranging from infections to cancer.
Multiple sclerosis (MS) is considered an autoimmune condition in which the
immune system attacks the central nervous system (CNS) causing demyelination.
The immune system is an extremely complex network of specialized cells and
organs that defends the body against attacks by "foreign" invaders such as
bacteria, viruses, fungi, and parasites. It accomplishes this by looking for and
destroying any invaders as they enter the body. Substances capable of triggering
an immune response or sounding "battle stations" are called antigens.
The immune system stockpiles a huge arsenal of cells, not only lymphocytes but
also cell-devouring phagocytes and their relatives. Some immune cells take on
all intruders, whereas others are trained on highly specific targets. To work
effectively, most immune cells need the cooperation of their comrades. Sometimes
immune cells communicate by direct physical contact, and sometimes they
communicate releasing chemical messengers.
The immune system shows both enormous diversity and extraordinary specificity.
It can recognize millions of distinctive foreign molecules and produce its own
molecules and cells to match up exactly with and counteract each of them.
In order to have room for enough cells to match the millions of possible foreign
invaders, the immune system stores just enough cells for each specific antigen.
When an antigen appears, those few specifically matched cells are activated and
stimulated so they will multiply into a full-scale army. Then to prevent this
army from over expanding after their job is complete, the immune system then
actives powerful mechanisms to suppress its own forces, the immune cells fade
away, leaving sentries behind to watch for future attacks.
Disorders of the immune system may occur when:
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The body generates an immune response against itself - autoimmune. |
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The body can't generate an appropriate
immune responses against invading microorganisms - immunodeficiency. |
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An excessive immune response to often harmless foreign antigens
that damages normal tissues - allergic. |
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The Lymphatic System
All immune cells come from precursors in the bone marrow and develop into mature
cells through a series of changes that can occur in different parts of the body.
Lymph nodes are a communication hub where immune cells sample information brought
in from the body. For instance, if adaptive immune cells in the lymph node recognize
pieces of a microbe brought in from a distant area, they will activate, replicate,
and leave the lymph node to circulate and address the pathogen. Thus, doctors may
check patients for swollen lymph nodes, which may indicate an active immune response.
The lymphatic system are the organs and are a vital part of the immune system
consisting of the thymus, bone marrow, spleen, tonsils, appendix, and Peyer's
patches in the small intestine. It's a network of lymph nodes connected by
lymphatic vessels that transports lymph throughout the body.
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The lymphatic system has three interrelated functions: it's responsible for the
removal of interstitial fluid from tissues; it absorbs and transports fatty
acids and fats as chyle to the circulatory system; and it transports immune
cells to and from the lymph nodes in to the sheppardian part of the bone. The
lymph transports antigen-presenting cells (APCs), such as dendritic cells, to
the lymph nodes where an immune response is stimulated. The lymph also carries
lymphocytes from the efferent lymphatics exiting the lymph nodes.
Lymph is formed from fluid that seeps through the thin walls of capillaries into
the body's tissues. This fluid contains oxygen, proteins, and other nutrients
that nourish the tissues. Some of this fluid reenters the capillaries and some
of it enters the lymphatic vessels (becoming lymph). Small lymphatic vessels
connect to larger ones and eventually form the thoracic duct. The thoracic duct
is the largest lymphatic vessel. It joins with the subclavian vein and thus
returns lymph to the bloodstream. The fluid also transports foreign substances
(such as bacteria), cancer cells, and dead or damaged cells that may be present
in tissues into the lymphatic vessels. Lymph also contains many white blood
cells.
All substances transported by the lymph pass through at least one lymph node,
where foreign substances can be filtered out and destroyed before fluid is
returned to the bloodstream. In the lymph nodes, white blood cells can collect,
interact with each other and antigens, and generate immune responses to foreign
substances. Lymph nodes contain a mesh of tissue that is tightly packed with B
lymphocytes (B cells), T lymphocytes (T cells), dendritic cells, and
macrophages. Harmful microorganisms are filtered through the mesh, then
identified and attacked by B and T cells.
The common lymphoid progenitor stem cell leads to adaptive immune cells (B and
T cells) that are responsible for mounting responses to specific microbes based on
previous encounters (immunological memory). Natural killer (NK) cells also are
derived from the common lymphoid progenitor and share features of both innate and
adaptive immune cells, as they provide immediate defenses like innate cells but also
may be retained as memory cells like adaptive cells. B, T, and NK cells also are
called lymphocytes.
Lymphocytes are one of the five kinds of white blood cells or leukocytes, which
are found circulating in the blood. Even though mature lymphocytes all look very
similar, they are very diverse in their functions. B cells are produced and
matured in the bone marrow.
The precursors of T cells leave the bone marrow and mature in the thymus which
is where they are processed and play a particularly important role in MS. They
travel continuously throughout the body patrolling for foreign invaders. In
order to recognize and respond to each specific antigen, each T cell's surface
carries special receptor molecules for particular antigens. Each B and T cell is
specific for a particular antigen, meaning that each is able to bind to a
specific molecular structure.
T cells contribute to the body's defenses in two major ways. Regulatory T cells
help keep in order or orchestrate the complex immune system. For instance, they
assist other cells to make antibodies, proteins programmed to match one specific
antigen much as a key matches a lock. Antibodies typically interact with
circulating antigens, such as bacteria, but are unable to penetrate living
cells. Chief among the regulatory T cells are those known as helper or inducer
cells. Helper T cells are essential for activating the body's defenses against
foreign substances. Another subset of regulatory T cells acts to turn off, or
suppress, various immune system cells when their job is done.
Organs of the Lymphatic System
The primary lymphoid organs are the sites where white blood cells are produced:
Bone marrow and the thymus. The secondary lymphoid organs include the spleen,
lymph nodes, tonsils, appendix, and Peyer's patches in the small intestine.
These organs trap microorganisms and other foreign substances and provide a
place for mature cells of the immune system to collect, interact with each other
and with the foreign substances, and generate a specific immune response.
Bone marrow
is the source of all the cells of the immune system. Cells form through a
process called hematopoiesis. During hematopoiesis, bone marrow-derived stem
cells differentiate into either mature cells of the immune system or into
precursors of cells that migrate out of the bone marrow to continue their
maturation elsewhere. The bone marrow produces all the different types of white
blood cells, including neutrophils, eosinophils, basophils, monocytes, B cells,
and the cells that develop into T cells (immature thymocytes), in addition to
red blood cells and platelets.
The thymus
function is to produce mature T cells. Immature thymocytes, also known as
prothymocytes, leave the bone marrow and migrate into the thymus. Through a
remarkable maturation process sometimes referred to as thymic education, T cells
that are beneficial to the immune system are spared, while those T cells that
might evoke a detrimental autoimmune response are eliminated. The mature T cells
are then released into the bloodstream. In the thymus, T cells are produced and
trained to recognize foreign antigens and to ignore the body's own antigens. T
cells are critical for any specific immunity.
The spleen
is essentially a large immunologic filter for blood. It's made up of B cells, T
cells, macrophages, dendritic cells, natural killer cells and red blood cells.
In addition to capturing foreign materials (antigens) from the blood that passes
through the spleen, migratory macrophages and dendritic cells bring antigens to
the spleen via the bloodstream. An immune response is initiated when the
macrophage or dendritic cells present the antigen to the appropriate B or T
cells. This organ can be thought of as an immunological conference center. In
the spleen, B cells become activated and produce large amounts of antibody.
Also, old red blood cells are destroyed in the spleen.
The lymph nodes
function as an immunologic filter for the bodily fluid known as lymph. Lymph
nodes are found throughout the body. Composed mostly of T cells, B cells,
dendritic cells and macrophages, the nodes drain fluid from most of our tissues.
Antigens are filtered out of the lymph in the lymph node before returning the
lymph to the circulation. In a similar fashion as the spleen, the macrophages
and dendritic cells that capture antigens present these foreign materials to T
and B cells, consequently initiating an immune response.
The lymph nodes are strategically placed in the body and are connected by an
extensive network of lymphatic vessels, which act as the immune system's
circulatory system. The lymphatic system transports microorganisms, other
foreign substances, cancer cells, and dead or damaged cells from the tissues to
the lymph nodes (which filter out and destroy these substances and cells), and
then to the bloodstream.
Lymph nodes are one of the first places that cancer cells can spread. Thus,
doctors often evaluate lymph nodes to determine whether a cancer has spread.
Cancer cells in a lymph node can cause the node to swell. Lymph nodes can also
swell after an infection because immune responses to infections are generated in
lymph nodes. Sometimes, bacteria that are carried to a lymph node are not killed
and cause an infection in the lymph node (lymphadenitis).
Self and Nonself
A healthy immune system has the ability to distinguish between the body’s own
cells, recognized as "self," and foreign cells, or "nonself." The body’s immune
defenses normally coexist peacefully with cells that carry distinctive "self"
marker molecules. But when immune defenders encounter foreign cells or organisms
carrying markers that say "nonself," they quickly launch an attack.
Killer T cells will strictly attack diseased or damaged body cells by binding to
them and bombarding them with lethal chemicals called cytokines. Since T cells
can attack cells directly, they must be able to discriminate between "self"
cells and "nonself" cells. To enable the immune system to distinguish its own
"self" cells from foreign cells, the cells in each body carry identifying
molecules on its surface. T cells likely to react against the "self" cells are
usually eliminated before leaving the thymus, and then remaining T cells
recognize the molecular markers and coexist peaceably with body tissues in a
state of self-tolerance.
The problem in autoimmune diseases such as MS, the peace
between the immune system and the body is disrupted when the immune system seems
to wrongly identify "self" as "nonself" and declares war on the body's myelin
that it no longer recognizes.
Researchers have looked for abnormalities or malfunctions in the blood/brain
barrier which is a protective membrane that controls the passage of substances
from the blood into the CNS. It's felt that in the case of MS, components of the
immune system get through the barrier and cause nervous system damage.
A number of infectious agents such as viruses that have been suspected of
causing MS, but researchers have been unable to implicate any one particular
agent. Viral infections are usually accompanied by inflammation and the
production of gamma interferon, a naturally occurring body chemical that has
been shown to worsen the clinical course of MS. It's possible that the immune
response to viral infections may themselves precipitate an MS attack. There
seems to be little doubt that something in the environment is somehow involved
in triggering MS.
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