Ingles III

IMMNUNE SYSTEM 

The role of the immune system is to protect against disease or other potentially damaging foreign bodies. When functioning properly, the immune system identifies a variety of threats, including viruses, bacteria and parasites, and distinguishes them from the body's own healthy tissue.

The major components of the immune system:

• Lymph nodes: Small, bean-shaped structures that produce and store cells that fight infection and disease and are part of the lymphatic system. Lymph nodes also contain lymph, the clear fluid that carries those cells to different parts of the body.

• Spleen: The largest lymphatic organ in the body, contains white blood cells that fight infection or disease. The spleen also helps control the amount of blood in the body and disposes of old or damaged blood cells.

• Bone marrow: The yellow tissue in the center of the bones produces white blood cells. This spongy tissue inside some bones, such as the hip and thigh bones, contains immature cells, called stem cells.

• Lymphocytes: These small white blood cells play a large role in defending the body against disease. The two types of lymphocytes are B-cells, which make antibodies that attack bacteria and toxins, and T-cells, which help destroy infected or cancerous cells. Killer T-cells are a subgroup of T-cells that kill cells that are infected with viruses and other pathogens or are otherwise damaged. Helper T-cells help determine which immune responses the body makes to a particular pathogen.

• Thymus: This small organ is where T-cells mature.

• Leukocytes: These disease-fighting white blood cells identify and eliminate pathogens and are the second arm of the innate immune system. The innate leukocytes include phagocytes (macrophages, neutrophils and dendritic cells), mast cells, eosinophils and basophils. 

When antigens are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce antibodies, which are specialized proteins that lock onto specific antigens.

Once produced, these antibodies stay in a person's body, so that if his or her immune system encounters that antigen again, the antibodies are already there to do their job. So if someone gets sick with a certain disease, like chickenpox, that person usually won't get sick from it again.

This is also how immunizations prevent certain diseases. An immunization introduces the body to an antigen in a way that doesn't make someone sick, but does allow the body to produce antibodies that will then protect the person from future attack by the germ or substance that produces that particular disease.

Although antibodies can recognize an antigen and lock onto it, they are not capable of destroying it without help. That's the job of the T cells, which are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. (Some T cells are actually called "killer cells.") T cells also are involved in helping signal other cells to do their jobs.

Antibodies also can neutralize toxins produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells.

BIBLIOGRAPHY: 
http://www.livescience.com/26579-immune-system.html
http://uhaweb.hartford.edu/BUGL/immune.htm