The Immune System

What is the immune system?

The purpose of the immune system is to keep infectious microorganisms, such as certain bacteria, viruses, and fungi, out of the body, and to destroy 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.

Anatomy of the immune system, adult
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The organs involved with the immune system are called the lymphoid organs, which 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)

  • appendix (a small tube that is connected to the large intestine)

  • blood vessels (the arteries, veins, and capillaries through which blood flows)

  • bone marrow (the soft, fatty 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 breast bone)

  • tonsils (two oval masses in the back of the throat)

What are lymphocytes?

Lymphocytes - a type of infection-fighting white blood cell - are vital to an effective immune system. Lymphocytes "patrol" the body for infectious microorganisms.

How are lymphocytes formed?

All cells, including immune cells such as lymphocytes, are produced in the bone marrow (the soft, fatty tissue found in bone cavities). 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 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 actually produce specific antibodies to specific infectious microorganisms, while T cells kill infectious microorganisms by killing the body cells that are affected. In addition, T cells release chemicals, called lymphokines, which trigger an immune response to combat cancer or a virus, for example.

Other types of white blood cells, such as phagocytes (engulfing cells) and cytotoxic cells (natural killer cells), actually kill the infectious microorganism by "devouring" it.

What is natural and acquired immunity?

The immune system has many different responsibilities. Not only does the immune system provide protection from infection through natural barriers, but it also adapts itself to provide immunity against infection by "remembering" the infectious microorganism from a previous exposure. The degree and duration of immunity depend on the type and amount of antigen and how it enters the body.

  • Natural immunity is created by the body's natural barriers, such as the skin, and protective substances in the mouth, the urinary tract, and on the eye surface. Another type of natural immunity is in the form of antibodies passed on from mother to child.

  • Acquired immunity develops through exposure to specific foreign microorganisms, toxins, and/or foreign tissues), which are "remembered" by the body's immune system. When that antigen enters the body again, the immune system "remembers" exactly how to respond to it, such as with chickenpox. Once a person is exposed to chickenpox or the chickenpox vaccine, the immune system will produce specific antibodies against chickenpox. When that same person is exposed to chickenpox again, the immune system will trigger the release of the particular chickenpox antibodies to fight the disease.

What are disorders of the immune system?

When the immune system does not function properly, it leaves the body susceptible to an array of diseases. 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:

  • cancer of the immune system

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

  • immune complex diseases, such as viral hepatitis and malaria

  • immunodeficiency diseases, such as acquired immunodeficiency syndrome (AIDS)

What is an infectious disease?

According to the Centers for Disease Control and Prevention (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 specific illness and country (some countries with poor community hygiene may still experience waterborne transmission of a disease), 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 the skin of an infected person

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

  • food-borne transmission - transmission of an infection through consuming 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.

What is the difference between viruses and bacteria?

Viruses and bacteria cause the majority of infections. Viruses cause most colds, flu, coughs, and sore throats. Bacteria cause most ear and sinus infections, strep throat, and urinary tract infections. Both bacteria and viruses can enter the body in many ways, including through inhalation, food, sexual contact, and skin contact.

How do antibiotics work against infections?

Antibiotics can be used to treat bacterial infections. However, antibiotics are ineffective in treating virus-related illnesses. In addition, antibiotics treat specific bacteria and 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.


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