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> > Immune System
The immune system is made up of special cells, proteins, tissues,
and organs that work together to defend people against organisms
and harmful substances that enter our systems and cause disease.
 The
organs of your immune system
The organs of your immune system are positioned throughout your
body, and work through a series of steps called the immune response.
They are called lymphoid organs because they are home to lymphocytes--the
white blood cells that are key operatives of the immune system.
Within these organs, the lymphocytes grow, develop, and are deployed.
Bone marrow, the soft tissue in the hollow center of bones, is
the ultimate source of all blood cells, including the immune cells.
The thymus is an organ that lies behind the breastbone; lymphocytes
known as T lymphocytes, or just T cells, mature there.
The spleen is a flattened organ at the upper left of the abdomen.
Like the lymph nodes, the spleen contains specialized compartments
where immune cells gather and confront antigens.
In addition to these organs, clumps of lymphoid tissue are found
in many parts of the body, especially in the linings of the digestive
tract and the airways and lungs--gateways to the body. These tissues
include the tonsils, adenoids, and appendix.
Humans have three types of immunity:
Innate Immunity
Everyone is born with innate (or natural) immunity, a type of general
protection that humans have. Many of the germs that affect other
species don't harm us. For example, the viruses that cause leukemia
in cats or distemper in dogs don't affect humans. Innate immunity
works both ways because some viruses that make humans ill - such
as the virus that causes HIV/AIDS - don't make cats or dogs sick
either.
Innate immunity also includes the external barriers of the body,
like the skin and mucous membranes (like those that line the nose,
throat, and gastrointestinal tract), which are our first line of
defense in preventing diseases from entering the body. If this outer
defensive wall is broken (like if you get a cut), the skin attempts
to heal the break quickly and special immune cells on the skin attack
invading germs.
Adaptive Immunity
We also have a second kind of protection called adaptive (or active)
immunity. This type of immunity develops throughout our lives. Adaptive
immunity involves the lymphocytes (as in the process described above)
and develops as children and adults are exposed to diseases or immunized
against diseases through vaccination.
Passive Immunity
Passive immunity is "borrowed" from another source
and it lasts for a short time. For example, antibodies in a mother's
breast milk provide an infant with temporary immunity to diseases
that the mother has been exposed to. This can help protect the infant
against infection during the early years of childhood.
Digestive System and the Immune System
The gastrointestinal tract is also a prominent part of the immune
system. The low pH (ranging from 1 to 4) of the stomach is fatal
for many harmful
Similarly, mucus (containing antibodies) neutralizes many of these
microorganisms.
Enzymes in the saliva and bile assist immune functions. Enzymes
such as Cyp3A4, along with the antiporter activities, are also instrumental
in the intestine's role of detoxification of antigens and xenobiotics,
such as drugs, involved in first pass metabolism.
Healthy intestinal bacteria serve to prevent the overgrowth of
potentially harmful bacteria in the gut.
Microorganisms are also kept at bay by an extensive immune system
comprising the gut-associated lymphoid tissue (GALT).
Things That Can Go Wrong With the Immune System
Disorders of the immune system can be broken down into four main
categories:
- immunodeficiency disorders (primary or acquired)
- autoimmune disorders (in which the body's own immune system
attacks its own tissue as foreign matter)
- allergic disorders (in which the immune system overreacts in
response to an antigen)
- cancers of the immune system
Immunodeficiency Disorders
Immunodeficiencies occur when a part of the immune system is not
present or is not working properly. Immunodeficiencies can affect
B lymphocytes, T lymphocytes, or phagocytes. Sometimes a person
is born with an immunodeficiency - these are called primary immunodeficiencies.
(Although primary immunodeficiencies are conditions that a person
is born with, symptoms of the disorder sometimes may not show up
until later in life.) Immunodeficiencies can also be acquired through
infection or produced by drugs. These are sometimes called secondary
immunodeficiencies.
Some examples of secondary immunodeficiencies are:
• HIV (human immunodeficiency virus) infection/AIDS (acquired
immunodeficiency syndrome) is a disease that slowly and steadily
destroys the immune system. It is caused by HIV, a virus which wipes
out certain types of lymphocytes called T-helper cells. Without
T-helper cells, the immune system is unable to defend the body against
normally harmless organisms, which can cause life-threatening infections
in people who have AIDS. Newborns can get HIV infection from their
mothers while in the uterus, during the birth process, or during
breastfeeding. People can get HIV infection by having unprotected
sexual intercourse with an infected person or from sharing contaminated
needles for drugs, steroids, or tattoos.
• Immunodeficiencies caused by medications. There are several
medicines that suppress the immune system. One of the drawbacks
of chemotherapy treatment for cancer, for example, is that it not
only attacks cancer cells, but other fast-growing, healthy cells,
including those found in the bone marrow and other parts of the
immune system. In addition, people with autoimmune disorders or
who have had organ transplants may need to take immunosuppressant
medications. These medicines can also reduce the immune system's
ability to fight infections and can cause secondary immunodeficiency.
Autoimmune Disorders
In autoimmune disorders, the immune system mistakenly attacks the
body's healthy organs and tissues as though they were foreign invaders.
Some examples of autoimmune diseases:
• Lupus is a chronic disease marked by muscle and joint
pain and inflammation. The abnormal immune response may also involve
attacks on the kidneys and other organs.
• Rheumatoid arthritis is a disease in which the body's immune
system acts as though certain body parts such as the joints of the
knee, hand, and foot are foreign tissue and attacks them.
• Scleroderma is a chronic autoimmune disease that can lead
to inflammation and damage of the skin, joints, and internal organs.
• Ankylosing spondylitis is a disease that involves inflammation
of the spine and joints, causing stiffness and pain.
Allergic Disorders
Allergic disorders occur when the immune system overreacts to exposure
to antigens in the environment. The substances that provoke such
attacks are called allergens.
The immune response can cause symptoms such as swelling, watery
eyes, and sneezing, and even a life-threatening reaction called
anaphylaxis.
Some examples of allergic disorders:
• Asthma, a respiratory disorder that can cause breathing
problems, frequently involves an allergic response by the lungs.
If the lungs are oversensitive to certain allergens (like pollen,
molds, animal dander, or dust mites), it can trigger breathing tubes
in the lungs to become narrowed, leading to reduced airflow and
making it hard for a person to breathe.
• Eczema is a scaly, itchy rash also known as atopic dermatitis.
Although atopic dermatitis is not necessarily caused by an allergic
reaction, it more often occurs in those who have allergies, hay
fever, or asthma or who have a family history of these conditions.
• Allergies of several types can occur. Environmental allergies
(to dust mites, for example), seasonal allergies (such as hay fever),
drug allergies (reactions to specific medications or drugs), food
allergies (such as to nuts), and allergies to toxins (bee stings,
for example) are the common conditions people usually refer to as
allergies.
Cancers of the Immune System
Cancer occurs when cells grow out of control. This can also happen
with the cells of the immune system. Lymphoma involves the lymphoid
tissues and Leukemia involves abnormal overgrowth of leukocytes.
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