Immune System


The immune system, which is made up of special cells, proteins,
tissues, and organs, defends people against germs and microorganisms
every day. In most cases, the immune system does a great job of keeping
people healthy and preventing infections. But sometimes problems with
the immune system can lead to illness and infection.

About the Immune System

The immune system is the body's defense against infectious organisms
and other invaders. Through a series of steps called the immune
response, the immune system attacks organisms and substances that invade
body systems and cause disease.

The immune system is made up of a network of cells, tissues, and
organs that work together to protect the body. The cells involved are
white blood cells, or leukocytes, which come in two basic types that
combine to seek out and destroy disease-causing organisms or substances.

Leukocytes are produced or stored in many locations in the body,
including the thymus, spleen, and bone marrow. For this reason, they're
called the lymphoid organs. There are also clumps of lymphoid tissue
throughout the body, primarily as lymph nodes, that house the
leukocytes.

The leukocytes circulate through the body between the organs and
nodes via lymphatic vessels and blood vessels. In this way, the immune
system works in a coordinated manner to monitor the body for germs or
substances that might cause problems.

The two basic types of leukocytes are:

1. phagocytes, cells that chew up invading organisms
   
2. lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them
   

A number of different cells are considered phagocytes. The most common type is the neutrophil,
which primarily fights bacteria. If doctors are worried about a
bacterial infection, they might order a blood test to see if a patient
has an increased number of neutrophils triggered by the infection. Other
types of phagocytes have their own jobs to make sure that the body
responds appropriately to a specific type of invader.

The two kinds of lymphocytes are B lymphocytes and T lymphocytes.
Lymphocytes start out in the bone marrow and either stay there and
mature into B cells, or they leave for the thymus gland, where they
mature into T cells. B lymphocytes and T lymphocytes have separate
functions: B lymphocytes are like the body's military intelligence
system, seeking out their targets and sending defenses to lock onto
them. T cells are like the soldiers, destroying the invaders that the
intelligence system has identified.

Here's how it works:

When antigens (foreign substances that invade the body) are detected,
several types of cells work together to recognize them and respond.
These cells trigger the B lymphocytes to produce antibodies, specialized
proteins that lock onto specific antigens.

Once produced, these antibodies continue to exist in a person's body,
so that if the same antigen is presented to the immune system again,
the antibodies are already there to do their job. So if someone gets
sick with a certain disease, like chickenpox, that person typically
doesn't get sick from it again.

This is also how immunizations prevent certain diseases. An
immunization introduces the body to an antigen in a way that doesn't
make someone sick, but does allow the body to produce antibodies that
will then protect the person from future attack by the germ or substance
that produces that particular disease.

Although antibodies can recognize an antigen and lock onto it, they
are not capable of destroying it without help. That's the job of the T
cells, which are part of the system that destroys antigens that have
been tagged by antibodies or cells that have been infected or somehow
changed. (Some T cells are actually called "killer cells.") T cells also
are involved in helping signal other cells (like phagocytes) to do
their jobs.

Antibodies also can neutralize toxins (poisonous or damaging
substances) produced by different organisms. Lastly, antibodies can
activate a group of proteins called complement that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells.

All of these specialized cells and parts of the immune system offer
the body protection against disease. This protection is called immunity.
Immunity

Humans have three types of immunity — innate, adaptive, and passive:

Innate Immunity

Everyone is born with innate (or natural) immunity, a type of general
protection. 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.

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 the first line of defense in
preventing diseases from entering the body. If this outer defensive wall
is broken (as through a cut), the skin attempts to heal the break
quickly and special immune cells on the skin attack invading germs.

Adaptive Immunity

The second kind of protection is adaptive (or active) immunity, which
develops throughout our lives. Adaptive immunity involves the
lymphocytes and develops as people 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 a
baby with temporary immunity to diseases the mother has been exposed
to. This can help protect the baby against infection during the early
years of childhood.

Everyone's immune system is different. Some people never seem to get
infections, whereas others seem to be sick all the time. As people get
older, they usually become immune to more germs as the immune system
comes into contact with more and more of them. That's why adults and
teens tend to get fewer colds than kids — their bodies have learned to
recognize and immediately attack many of the viruses that cause colds.Problems of the Immune System

Disorders of the immune system fall into into four main categories:

1. immunodeficiency disorders (primary or acquired)
   
2. autoimmune disorders (in which the body's own immune system attacks its own tissue as foreign matter)
   
3. allergic disorders (in which the immune system overreacts in response to an antigen)
   
4. cancers of the immune system
   

Immunodeficiency Disorders


Immunodeficiencies occur when a part of the immune system is not
present or is not working properly. Sometimes a person is born with an
immunodeficiency (known as primary immunodeficiencies), although
symptoms of the disorder might not appear until later in life.
Immunodeficiencies also can be acquired through infection or produced by
drugs (these are sometimes called secondary immunodeficiencies).

Immunodeficiencies can affect B lymphocytes, T lymphocytes, or
phagocytes. Examples of primary immunodeficiencies that can affect kids
and teens are:

• IgA deficiency is the most common immunodeficiency
  disorder. IgA is an immunoglobulin that is found primarily in the saliva
  and other body fluids that help guard the entrances to the body. IgA
  deficiency is a disorder in which the body doesn't produce enough of the
  antibody IgA. People with IgA deficiency tend to have allergies or get
  more colds and other respiratory infections, but the condition is
  usually not severe.
  
• Severe combined immunodeficiency (SCID) is also
  known as the "bubble boy disease" after a Texas boy with SCID who lived
  in a germ-free plastic bubble. SCID is a serious immune system disorder
  that occurs because of a lack of both B and T lymphocytes, which makes
  it almost impossible to fight infections.
  
• DiGeorge syndrome (thymic dysplasia), a birth
  defect in which kids are born without a thymus gland, is an example of a
  primary T-lymphocyte disease. The thymus gland is where T lymphocytes
  normally mature.
  
• Chediak-Higashi syndrome and chronic granulomatous disease both involve the inability of the neutrophils to function normally as phagocytes.
  

Acquired (or secondary) immunodeficiencies usually develop after
someone has a disease, although they can also be the result of
malnutrition, burns, or other medical problems. Certain medicines also
can cause problems with the functioning of the immune system.

Acquired (secondary) immunodeficiencies include:

• 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 that 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. Some
  medicines 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, which also
  can reduce the immune system's ability to fight infections and can cause
  secondary immunodeficiency.
  

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Autoimmune Disorders


In autoimmune disorders, the immune system mistakenly attacks the
body's healthy organs and tissues as though they were foreign invaders.
Autoimmune diseases include:

• Lupus, a chronic disease marked by muscle and joint
  pain and inflammation (the abnormal immune response also may involve
  attacks on the kidneys and other organs)
  
• Juvenile rheumatoid arthritis, 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, a chronic autoimmune disease that can lead to inflammation and damage of the skin, joints, and internal organs
  
• Ankylosing spondylitis, a disease that involves inflammation of the spine and joints, causing stiffness and pain
  
• Juvenile dermatomyositis, a disorder marked by inflammation and damage of the skin and muscles
  

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. Medications called
antihistamines can relieve most symptoms.

Allergic disorders include:

• 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 an itchy rash also known as atopic
  dermatitis. Although atopic dermatitis is not necessarily caused by an
  allergic reaction, it more often occurs in kids and teens who have
  allergies, hay fever, or asthma or who have a family history of these
  conditions.
  
• Allergies of several types can occur in kids and
  teens. 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 also can happen
with the cells of the immune system. Lymphoma involves the lymphoid
tissues and is one of the more common childhood cancers. Leukemia, which
involves abnormal overgrowth of leukocytes, is the most common
childhood cancer. With current medications most cases of both types of
cancer in kids and teens are curable.

Although immune system disorders usually can't be prevented, you can
help your child's immune system stay stronger and fight illnesses by
staying informed about your child's condition and working closely with
your doctor.