Immune System

Encyclopedia of Childhood and Adolescence

The immune system provides the human body with protection from the microorganisms that cause disease. Traditionally scientists viewed the immune system as a defensive network that protected the "self" from infectious "non-self" invaders. In the mid-1990s, some immunologists modified this view of the immune system, creating a new model of the body's immune system that is able to discriminate between beneficial "non-self" invaders (food or helpful bacteria) and threatening invaders. One of the leading scientists investigating the functioning of the immune system in the 1990s was Polly Matzinger of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. Matzinger proposed a model of the immune system that responds to invaders only when cells of the body are injured or damaged.

No matter what model is used, immunologists generally agree that the immune systems consists of three lines of defense. The first line is made up of the physical barriers--the skin and mucous membranes--that prevent microorganisms from entering the body. The next line of defense, the innate or non-specific immunity, features responses from cells that surround and digest invaders, and from chemicals like histamine and serum proteins that help to destroy bacteria. The final defense is slower acting but more specific to the invader. This specific immunity calls into action the lymphocytes or white blood cells produced by the thymus and bone marrow.

The human body is constantly bombarded with microorganisms, including viruses (such as those that cause colds and influenza), bacteria (such as those that cause pneumonia and food poisoning), parasites, and fungi. The immune system efficiently wages a daily battle to rid the body of harmful organisms. When the immune system is unable to function because of injury or damage, the consequences are severe. For instance, Acquired Immune Deficiency Syndrome (AIDS) is caused by a virus--human immunodeficiency virus (HIV)--that attacks a key immune system cell, the helper T-cell lymphocyte. Without these cells, the immune system cannot fight off the harmful microorganisms. Eventually, the person succumbs to infections that a healthy immune system would effortlessly neutralize.

The organs of the immune system either make the cells that participate in the immune response or act as sites for immune function. These organs include the lymphatic vessels, lymph nodes, tonsils, thymus, Peyer's patches, and spleen. Lymphatic fluid (or lymph) circulates through the lymph nodes via the lymphatic vessels. The lymph nodes are small aggregations of tissues located throughout the lymphatic system. White blood cells (lymphocytes) that function in the immune response are concentrated in the lymph nodes where foreign cells of microorganisms are detected and overpowered.

The tonsils and Peyer's patches contain large numbers of lymphocytes. Located at the back of the throat and under the tongue (tonsils) and in the small intestine and appendix (Peyer's patches), these organs filter out potentially harmful bacteria that may enter the body via the nose, mouth, and digestive system.

The thymus gland, located within the upper chest region, weighs about 15 grams or one-half ounce at birth. It continues to grow until, by the time the child has reached age 12, the thymus has roughly doubled in size. During childhood, the thymus makes large numbers of the lymphocytes known as T-lymphocytes or T-cells. Around puberty, T-cell production is taken over by the lymph nodes and spleen, and the thymus begins to shrink. By adulthood, it is sometimes impossible to detect in x rays. Prior to puberty , removal of the thymus due to disease or injury in a child may have a negative effect on both physical growth and the development of immunity to certain organisms.

Bone marrow, found within the interior of bones, also produces lymphocytes that migrate out of the bone marrow to other sites in the body. Because bone marrow is an integral part of the immune system, certain bone cancer treatments that require the destruction of bone marrow are extremely risky, because without bone marrow, a person cannot make lymphocytes. People undergoing bone marrow replacement must be kept in strict isolation to prevent exposure to viruses or bacteria.

The spleen destroys worn-out red blood cells and acts as a reservoir for blood. Any rupture to the spleen can cause dangerous internal bleeding, a potentially fatal condition. The spleen also contains lymphatic tissue and produces lymphocytes.

For the immune system to work properly, two things must happen: first, the body must recognize that it is being threatened by foreign microorganisms. Second, the immune response must be quickly activated before many body tissue cells are destroyed by the invaders.

The skin and mucous membranes act as effective barriers against harmful invaders. The surface of the skin is slightly acidic which makes it difficult for many microorganisms to survive. In addition, the enzyme lysozyme, present in sweat, tears, and saliva, kills many bacteria. Mucous membranes line many of the body's entrances, such as those that open into the respiratory, digestive, and uro-genital tract. Bacteria become trapped in the thick mucous layers and are thus prevented from entering the body.