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. One of the important cells involved are white blood
cells, also called 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:
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.
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
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, which are specialized proteins that lock onto
Once produced, these antibodies stay in a person's body, so that if his or her immune
system encounters that antigen again, the antibodies are already there to do their
job. So if someone gets sick with a certain disease, like chickenpox, that person
usually won'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
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.
Humans have three types of immunity — innate, adaptive, and passive:
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.
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 is "borrowed" from another source and it lasts for a short time.
For example, antibodies in a mother's breast milk give a baby 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
Problems of the Immune System
Disorders of the immune system fall 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
Immunodeficiencies happen when a part of the immune system is missing or not working
properly. Some people are 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(CGD) 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
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.
In autoimmune disorders, the immune system mistakenly attacks the body's healthy
organs and tissues as though they were foreign invaders. Autoimmune diseases include:
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)
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
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 happen 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.
Medicines called antihistamines can relieve most symptoms.
Allergic disorders include:
a respiratory disorder that can cause breathing problems, often involves an allergic
response by the lungs. If the lungs are oversensitive to certain allergens (like pollen,
molds, animal dander, or dust mites), breathing tubes can become narrowed and swollen,
making it hard for a person to breathe.
is an itchy rash also known as atopic dermatitis. Although not necessarily caused
by an allergic reaction, eczema most often happens in kids and teens who have allergies,
hay fever, or asthma or who have a family history of these conditions.
of several types can affect 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 happens when cells grow out of control. This can include cells of the
immune system. Leukemia,
which involves abnormal overgrowth of leukocytes, is the most common childhood cancer.
the lymphoid tissues and is also one of the more common childhood cancers. With current
treatments, 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.