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
What the Immune System Does
The immune system is the body's defense against infectious
organisms and other invaders. Through a series of steps called the
, the immune system attacks organisms and substances that invade
our systems and cause disease. The immune system is made up of a
network of cells, tissues, and organs that work together to protect
The cells that are part of this defense system are white blood
. They come in two basic types (more on these below), which combine
to seek out and destroy the organisms or substances that cause
Leukocytes are produced or stored in many locations throughout
the body, including the thymus, spleen, and bone marrow. For this
reason, they are called the
organs. There are also clumps of lymphoid tissue throughout the
body, primarily in the form of lymph nodes, that house the
The leukocytes circulate through the body between the organs and
nodes by means of the
. Leukocytes can also circulate through the 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:
, cells that chew up invading organisms
, 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
, 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
There are two kinds of lymphocytes: the
. 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 jobs to do: 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.
are foreign substances that invade the body. When an antigen is
detected, several types of cells work together to recognize and
respond to it. These cells trigger the B lymphocytes to produce
antibodies, specialized proteins that lock onto specific antigens.
Antibodies and antigens fit together like a key and a lock.
Once the B lymphocytes have produced antibodies, 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. That's why if
someone gets sick with a certain disease, like chickenpox, that
person typically doesn't get sick from it again. This is also
why we use immunizations to prevent getting certain diseases. The
immunization introduces the body to the antigen in a way that
doesn't make a person sick, but it does allow the body to
produce antibodies that will then protect that person from future
attack by the germ or substance that produces that particular
Although antibodies can recognize an antigen and lock onto it,
they are not capable of destroying it without help. That is the job
of the T cells. The T cells are part of the system that destroys
antigens that have been tagged by antibodies or cells that have
been infected or somehow changed. (There are actually T cells that
are called "killer cells.") T cells are also involved in
helping signal other cells (like phagocytes) to do their jobs.
Antibodies can also neutralize toxins (poisonous or damaging
substances) produced by different organisms. Lastly, antibodies can
activate a group of proteins called
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
Humans have three types of immunity - innate, adaptive, and
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.
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
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
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.
Things That Can Go Wrong With the Immune System
Disorders of the immune system can be broken down into four main
- 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
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 - 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
Immunodeficiencies can affect B lymphocytes, T lymphocytes, or
phagocytes. Some examples of primary immunodeficiencies that can
affect kids and teens are:
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
DiGeorge syndrome (thymic dysplasia)
, a birth defect in which children are born without a thymus
gland, is an example of a primary T-lymphocyte disease. The
thymus gland is where T lymphocytes normally mature.
chronic granulomatous disease
both involve the inability of the neutrophils to function
normally as phagocytes.
Acquired immunodeficiencies usually develop after a person 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. Secondary
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
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. These medicines can also reduce the immune
system's ability to fight infections and can cause secondary
In autoimmune disorders, the immune system mistakenly attacks
the body's healthy organs and tissues as though they were
foreign invaders. Autoimmune diseases include:
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.
Juvenile 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.
is a chronic autoimmune disease that can lead to inflammation and
damage of the skin, joints, and internal organs.
is a disease that involves inflammation of the spine and joints,
causing stiffness and pain.
is a disorder marked by inflammation and damage of the skin and
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. Taking
medications called antihistamines can relieve most symptoms.
Allergic disorders include:
, 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.
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
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 can also
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.
Steven Dowshen, MD
Date reviewed: November 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
© 1995-2009 The Nemours Foundation/KidsHealth. All rights reserved.