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.
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 immune response,
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 body.
The cells that are part of this defense system are white blood cells, or
They come in two basic types (more on these below), which combine to seek out
and destroy the organisms or substances that cause disease.
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 lymphoid 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 lymphatic vessels. 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
The two basic types of leukocytes are:
- phagocytes, cells that chew up invading organisms
- 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.
There are two kinds of lymphocytes: the B lymphocytes and
the 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
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.
Antigens 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 disease.
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 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.
Humans have three types of immunity — innate, adaptive, and passive:
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
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 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
- 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 secondary immunodeficiencies.
Immunodeficiencies can affect B lymphocytes, T lymphocytes, or phagocytes.
Some 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 children are born without a thymus gland, is an example of a primary
T-lymphocyte disease. The thymus gland is where T lymphocytes normally
- Chediak-Higashi syndrome and 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 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 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. 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
- 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.
- 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.
- 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.
- Juvenile dermatomyositis is a disorder marked by
inflammation and damage of the skin and muscles.
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
- 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
- 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
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.