What is the immune system?

The immune system keeps infectious microorganisms, such as bacteria, viruses, and fungi, out of the body. It also destroys any infectious microorganisms that do invade the body. The immune system is made up of a complex and vital network of cells and organs that protect the body from infection.

The organs involved with the immune system are called the lymphoid organs. They affect growth, development, and the release of lymphocytes (a certain type of white blood cell). The blood vessels and lymphatic vessels are important parts of the lymphoid organs because they carry the lymphocytes to and from different areas in the body. Each lymphoid organ plays a role in the production and activation of lymphocytes. Lymphoid organs include:

• Adenoids. Two glands located at the back of the nasal passage.

• Bone marrow. The soft, spongy tissue found in bone cavities.

• Lymph nodes. Small organs shaped like beans, which are located throughout the body and connect via the lymphatic vessels.

• Lymphatic vessels. A network of channels throughout the body that carries lymphocytes to the lymphoid organs and bloodstream.

• Peyer's patches. Lymphoid tissue in the small intestine.

• Spleen. A fist-sized organ located in the abdominal cavity.

• Thymus. Two lobes that join in front of the trachea behind the breastbone.

• Tonsils. Two oval masses in the back of the throat.

What are lymphocytes?

Lymphocytes are a type of infection-fighting white blood cell that are vital to an effective immune system.

How are lymphocytes formed?

The precursors of all blood cells, including immune cells, such as lymphocytes, are produced in the bone marrow. Certain cells will become part of the group of lymphocytes, while others will become part of another type of immune cells known as phagocytes. Once the lymphocytes are initially formed, some will continue to mature in the bone marrow and become "B" cells. Other lymphocytes will finish their maturation in the thymus and become "T" cells. B and T cells are the two major groups of lymphocytes that recognize and attack infectious microorganisms.

Once mature, some lymphocytes will be housed in the lymphoid organs, while others will travel continuously around the body through the lymphatic vessels and bloodstream.

How do lymphocytes fight infection?

Although each type of lymphocyte fights infection differently, the goal of protecting the body from infection remains the same. B cells produce specific antibodies to infectious microorganisms. T cells kill infectious microorganisms by killing the body cells that are affected. T cells also release chemicals called cytokines.

Other types of white blood cells, such as phagocytes (engulfing cells) and natural killer cells (cytotoxic cells), actually destroy the infectious microorganisms.

What are disorders of the immune system?

When the immune system does not function properly, a number of diseases can occur. Allergies and hypersensitivity to certain substances are considered immune system disorders. In addition, the immune system plays a role in the rejection process of transplanted organs or tissue. Other examples of immune disorders include:

• Autoimmune diseases, such as juvenile diabetes, rheumatoid arthritis, and anemia

• Immunodeficiency diseases, such as acquired immune deficiency syndrome (AIDS) and severe combined immunodeficiency (SCID).

What is an infectious disease?

According to the CDC, an infectious disease is caused by one, or more, of the following:

• Viruses

• Bacteria

• Parasites

• Fungi

Infectious diseases can range from common illnesses, such as the cold, to deadly illnesses, such as AIDS. Depending on the disease-causing organism, an infection can spread in some, or all, of the following ways:

• Sexual transmission through sexual contact, including intercourse.

• Airborne transmission through inhaling airborne droplets of the disease, which may exist in the air as a result of a cough or sneeze from an infected person.

• Blood-borne transmission through contact with infected blood, such as when sharing hypodermic needles.

• Direct contact transmission through a direct body surface-to-body contact with an infected person.

• Insect-borne transmission through insects, such as mosquitoes, which draw blood from an infected person and then bite a healthy person.

• Food-borne transmission through consuming contaminated food.

• Water-borne transmission through contact with contaminated water.

• Other mechanisms that can transmit a disease.

In developed countries, most infections are spread through sexual, airborne, blood-borne, and direct contact transmission.

How do antibiotics work against infections?

Antibiotics can be used to treat bacterial infections. However, antibiotics are ineffective in treating illnesses caused by viruses. In addition, antibiotics treat specific bacteria. Overuse or misuse of antibiotics can lead to drug-resistant bacteria. It is important that antibiotics are taken properly and for the duration of the prescription. If antibiotics are stopped early, the bacteria may develop a resistance to the antibiotics and the infection may reoccur.