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.
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:
Lymphocytes are a type of infection-fighting white blood cell which are vital to an effective immune system.
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 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.
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, while T cells kill infectious microorganisms by killing the body cells that are affected. In addition, T cells 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.
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:
According to the Centers for Disease Control and Prevention (CDC), an infectious disease is caused by one, or more, of the following:
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:
In developed countries, most infections are spread through sexual, airborne, blood-borne, and direct contact transmission.
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.
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Online Resources of Infectious Diseases