The immune system has two major functions: 1) to recognize
substances (also called antigens) that are foreign to the body and 2) to
react against them. These foreign substances (or antigens) may be
microorganisms that cause infectious diseases, transplanted tissues or
organs from another individual, or even "foreign" tumors that arise within
one's own body. Adequate functioning of the immune system provides
protection from infectious diseases, is responsible for the rejection of
transplanted organs, and may protect the individual from the development of
cancer.
One of the most important functions of the immune system
is to protect against infectious diseases. The body is constantly challenged
by a variety of infectious microorganisms such as bacteria, viruses, and
fungi. These microorganisms can cause a variety of infections - some
relatively common and usually not serious, and others less common and more
serious. For example, the average individual has a number of "colds" each
year caused by a variety of different respiratory viruses. Other viruses can
cause more serious liver infections (hepatitis) or brain infections
(encephalitis). Common bacterial infections include "strep" throats, some
skin infections (impetigo) and ear infections (otitis). Occasionally a
bacterial infection may be very serious as when it affects the covering of
the brain (meningitis) or involves the bone (osteomyelitis) or joints
(pyogenic arthritis).
Whatever the infection, whether it is caused by a
bacterium, a virus or a fungus, whether it is relatively harmless or
relatively serious, whether it is in the skin, the throat, the lungs, or the
brain, the immune system is responsible for defending the individual against
the invading microorganism. A normal immune system provides a person with
the ability to kill the invading microorganism, limit the spread of
infection, and ultimately recover. An abnormal immune system is unable to
kill the microorganism effectively. The infection may spread and, if
untreated, the individual may ultimately die from the infection. Thus,
patients with a defective immune system often have an increased
susceptibility to infection as one of their major problems. In some
individuals, the infections may occur infrequently and be of relatively
little consequence. In others, the infections maybe unusually frequent,
unusually severe, or caused by unusual and/or rare microorganisms.
Location of the Immune System in the Body
Because all parts of the body need to be protected from
microorganisms or other foreign material, the immune system is located in,
or has access to, virtually all parts of the body. Thus, important
components of the immune system are concentrated in the blood, thymus, lymph
nodes, bone marrow, spleen, tonsils, adenoids, lungs, liver, and intestines.
In addition, when an infection starts in a location that has only a few
components of the immune system, such as the skin, signals are sent
throughout the body to call in large numbers of immune cells to the site of
infection.
Components of the Immune System
The immune system is composed of a variety of different
cell types and proteins. Each component performs a special task aimed at
recognizing foreign material (antigens_ and/or reacting against foreign
material. For some components, recognition of the material as foreign to the
body is their primary and only function. Other components function primarily
to react against the foreign material. Still other components function to
both recognize and react against foreign antigens.
Since the functions of the immune system are so critical
to survival, many of them can be performed by more than one component of the
system. This redundancy acts as a backup mechanism so that if one component
of the whole system is missing or functioning poorly another component can
take over at least some of its functions.
The major components of the immune system are:
B-lymphocytes
T-lymphocytes
Phagocytes
Complement
B-lymphocytes: B-lymphocytes (sometimes called B cells)
are specialized cells of the immune system whose major function is to
produce antibodies (also called immunoglobulins or gammaglobulins).
B-lymphocytes develop from primitive cells (stem cells) in the bone marrow.
When mature, B-lymphocytes can be found in the bone marrow, lymph nodes,
spleen, certain areas of the intestine, and to a lesser extent in the
bloodstream.
When B-lymphocytes are stimulated by a foreign material
(antigens), they respond by maturing into another cell type called plasma
cells. The plasma cells produce antibodies. Antibodies, the major product of
plasma cells, find their way into the bloodstream, respiratory secretions,
the intestinal secretions, and even tears.
Antibodies are highly specialized serum protein molecules.
For every foreign antigen, there are antibody molecules specifically
designed for that antigen. Thus, there are antibody molecules that fit, like
a lock and key, the polio virus, others that are aimed specifically at the
bacteria that causes diphtheria, and still others that match the measles
virus. The variety of different antibody molecules is so extensive that
B-lymphocytes have the ability to produce them against virtually all
possible microorganisms in our environment. When antibody molecules
recognize the microorganism as foreign, they physically attach to the
microorganism and set off a complex chain of reactions involving other
components of the immune system that eventually destroys the microorganism.
The chemical names for antibody proteins are
"immunoglobulins" of "gammaglobulins". Just as antibodies can vary from
molecule to molecule with respect to which microorganisms they bind, they
can also vary with respect to their specialized functions in the body. This
kind of variation in specialized function is determined by the antibody's
chemical structure, which in turn determines the class of the antibody ( or
immunoglobulin). There are 5 major classes of antibodies or immunoglobulins:
Immunoglubulin G (IgG)
Immunoglubulin A (IgA)
Immunoglubulin M (IgM)
Immunoglubulin E (IgE)
Immunoglubulin D (IgD)
Each immunoglobulin class has special chemical
characteristics which provide it with certain advantages. For example,
antibodies in the IgG fraction are formed in large quantities and can travel
from the bloodstream to the tissues. THese immunoglobulins (antibodies) are
the only class of immunoglobulins which cross the placenta and pass immunity
from the mother to the newborn. Antibodies of the IgA fraction are produced
near mucus membranes and find their way into secretions such as tears, bile,
saliva, and mucus, where they protect against infection in the respiratory
tract and intestines. Antibodies of the IgM class are the first antibodies
formed in response to infection and, therefore, are important in protection
during the first few days of an infection. Antibodies of the IgE class are
responsible for allergic reactions. The specialized function of IgD is still
not completely understood.
Antibodies protect the host against infection in a number
of different ways. For example, some microorganisms must attach to body
cells before they can cause an infection, but antibody on the surface of a
microorganism can interfere with the microorganism's ability to adhere to
the host cell. In addition, antibody attached to the surface of some
microorganisms can cause the activation of a group of proteins called the
complement system which can directly kill the bacteria which are not coated
with antibody. All of these actions of antibodies prevent microorganisms
from successfully invading body tissues where they may cause serious
infections.
T-lymphocytes: T-lymphocytes (sometimes called T-cells)
are another type of immune cell. T-lymphocytes do not produce antibody
molecules. The specialized roles of T-lymphocytes are 1) to directly attack
foreign antigens such as viruses, fungi, or transplanted tissues, and 2) to
act as regulator of the immune system.
T-lymphocytes develop from stem cells in the bone marrow.
Early in fetal life, the immature cells migrate to they thymus, a
specialized organ of the immune system in the chest. Within the thymus,
immature lymphocytes develop into mature T-lymphocytes ("T" for the thymus).
The thymus is essential for this process, and T-lymphocytes can not develop
if the fetus has no thymus. Mature T-lymphocytes leave the thymus and
populate other organs of the immune system, such as the spleen, lymph nodes,
bone marrow, and blood.
Each T-lymphocyte reacts with a specific antigen, just as
each antibody molecule reacts with a specific antigen. In fact,
T-lymphocytes have molecules on their surfaces that are like antibodies and
recognize antigens. The variety of different T-lymphocytes is so extensive
that the body has T-lymphocytes which can react against virtually any
antigen. T-lymphocytes also vary with respect to their function. There are
1) "killer" of "effector" T-lymphocytes, 2) "helper" T-lymphocytes, y 3)
"suppressor" T-lymphocytes. Each has a different role to play in the immune
system.
Killer or effector T-lymphocytes are the T-lymphocytes
which perform the actual destruction of the invading microorganism. These
killer T-lymphocytes protect the body from certain bacteria and viruses
which have the ability to survive and even reproduce within the body's own
cells. Killer T-lymphocytes also respond to foreign tissues in the body,
such as a transplanted kidney. The killer T-lymphocytes migrate to the site
of an infection or the transplanted tissues. Once there, the killer cell
directly binds to its target and kills it.
Helper T-lymphocytes assist B-lymphocytes in producing
antibody and assist killer T-lymphocytes in their attack on foreign
substances. The helper T-lymphocytes helps or enhances the function of
B-lymphocytes, causing them to produce more antibodies more quickly. Helper
T-lymphocytes also help or enhance the function of killer T-lymphocytes.
Conversely supressor T-lymphocytes suppress or turn off helper
T-lymphocytes. Without the suppressor cells, the immune system would keep
working even after an infection had been cured. Together, helper and
suppressor T-lymphocytes act as the thermostat of the entire lymphocyte
system to keep it turned on just enough - not too much and not too little.
Phagocytes: Phagocytes are specialized cells of the immune
system whose primary function is to ingest and kill microorganisms. These
cells, like the others in the immune system, develop from primitive "stem"
cells in the bone marrow. When mature, they migrate to virtually all tissues
of the body but are especially prominent in the bloodstream, spleen, liver,
lymph nodes, and lungs.
There are a number of different types of phagocytic cells.
Polymorphonuclear leukocytes (neutrphils or granylocytes) are commonly found
in the bloodstream and can migrate into sites of infection within a matter
of minutes. It is this phagocytic cell that increases in number in the blood
during infection and is in large part responsible for an elevated white
blood cell count during infection. It also is the phagocytic cell that
leaves the bloodstream and accumulates in the tissues during the first few
hours of infection, and is responsible for the formation of "pus". Monocytes
are another type of phagocytic cell found circulating in the bloodstream.
They also line the walls of blood vessels in organs like the liver and
spleen. Here they act to capture microorganisms as they pass by in the
blood. When monocytes leave the bloodstream and enter the tissues, they
change shape and size and become macrophages.
Phagocytic cells serve a number of critical functions in
the body's defense against infection. They have the ability to leave the
bloodstream and move in to the tissues to the site of infection. Once at the
site of infection, they ingest the invading microorganism. Ingestion of
microorganisms by phagocytic cells is made easier when the microorganisms
are coated with either antibody or complement or both. Once the phaygocytic
cell gas engulfed or ingested the microorganism, it initiates a series of
chemical reactions within the cell which result in the death of the
microorganism.
Complement: The complement system is composed of 18 serum
proteins, which function in an ordered and integrated fashion to help defend
against infection and produce inflammation. Some of the proteins in the
complement system are produced in the liver, while others are produced by
certain phagocytic cells, the macrophages.
In order to perform their protective functions, the
complement components must be converted from inactive forms to activated
forms. In some instances, microorganisms must first combine with antibody in
order to activate complement. In other cases, the microorganisms can
activate complement without the need for antibody. Once activated, the
complement system can perform a number of important functions in defense
against infection. As mentioned above, one of the proteins of the complement
system coats microorganisms to make them more easily infested by phagocytic
cells. Other components of complement act to send put chemical signals to
attract phagocytic cells to the sites of infection. When the whole system is
assembled on the surface of some microorganisms, a complex is created which
can puncture the cell membrane, or outer envelope, of the microorganism and
kill it.
