The Immune System

What is the immune system?

The immune system works to keep germs out of the body and destroy any that get in. The immune system is made up of a complex network of cells and organs that protect the body from infection.

Illustration of the immune system

Lymph nodes are part of the immune system. They release lymphocytes, a certain type of white blood cell that fights infection. The blood vessels and lymph vessels carry the lymphocytes to and from different areas in the body. Each lymphoid organ plays a role in the production and activation of lymphocytes. Organs in the lymph system include:

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

  • Blood vessels. The arteries, veins, and capillaries through which blood flows.

  • Bone marrow. The soft, fatty tissue found inside bones where blood cells are made.

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

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

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

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

  • Thymus. Two lobes that join in front of the trachea (windpipe) 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. They are vital to an effective immune system.

How are lymphocytes formed?

Blood cells and immune cells are made 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 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 which 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. The B cells produce specific antibodies to fight infectious microorganisms, while T cells kill infectious microorganisms by killing the body cells that are affected. In addition, T cells release chemicals called cytokines, which are cellular messengers. 

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

What are disorders of the immune system?

When the immune system does not work properly, it leaves the body at risk for disease. Allergies and hypersensitivity to certain substances are considered immune system disorders. In addition, the immune system plays a role in the rejection of transplanted organs or tissue. Other examples of immune disorders include:

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

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

What is an infectious disease?

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 infectious disease can spread in some, or all, of the following ways:

  • Sexual transmission. Transmission of an infection through sexual contact, including intercourse.

  • Airborne transmission. Transmission of an infection 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. Transmission of an infection through contact with infected blood, such as when sharing hypodermic needles.

  • Direct skin contact. Transmission of an infection through contact with an infected area on the skin.

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

  • Foodborne transmission. Transmission of an infection through eating contaminated food.

  • Water-borne transmission. Transmission of an infection 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 skin 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.