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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
The Immune System - Click to Enlarge

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 all over the body and connect via the lymphatic vessels.

  • Lymph vessels. A network of channels all over 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 belly.

  • Thymus. A small organ 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. Others will become part of another type of immune cell known as phagocytes. Once the lymphocytes are formed, some will continue to grow in the bone marrow and become B cells. Other lymphocytes will finish growing in the thymus and become T cells. B and T cells are the 2 major groups of lymphocytes which recognize and attack infectious microorganisms.

Once mature, some lymphocytes will stay in the lymphoid organs. Others will keep moving around the body through the lymphatic vessels and bloodstream.

How do lymphocytes fight infection?

Each type of lymphocyte fights infection differently. But the goal of protecting the body from infection remains the same. The B cells make specific antibodies to fight infectious microorganisms. The T cells kill infectious microorganisms by killing the body cells that are affected. T cells also release chemicals (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 being very sensitive to certain substances are both considered immune system disorders. In addition, the immune system plays a role in rejecting 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 1 or more of the following:

  • Viruses

  • Bacteria

  • Parasites

  • Fungi

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

  • Sexual contact, including intercourse

  • Breathing in airborne droplets of the disease after an infected person coughs or sneezes

  • Contact with infected blood, such as when sharing hypodermic needles

  • Contact with an infected area on the skin

  • Insects (such as mosquitoes or ticks) which draw blood from an infected host and then bite a healthy person

  • Eating contaminated food

  • Contact with contaminated water

  • Other methods that can transmit a disease

In developed countries, most infections are spread through sexual contact, airborne, bloodborne, and direct skin contact transmission.

How do antibiotics work against infections?

Antibiotics can be used to treat bacterial infections. But antibiotics are not effective 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 you stop taking antibiotics early, the bacteria may become resistant to the antibiotics. The infection may reoccur.

Online Medical Reviewer: Lentnek, Arnold, MD
Online Medical Reviewer: Sather, Rita, RN
Date Last Reviewed: 10/1/2016
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