Immune System & Defenses – Part 1

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Immune System

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.

Organs of the immune system

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.

Overview of the immune system

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.

Barriers: skin and mucous membranes

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.

In the upper respiratory tract, the hairs that line the nose also trap bacteria. Any bacteria that are inhaled deeper into the respiratory tract are swept back out again by the cilia--tiny hairs--that line the trachea and bronchii. One reason why smokers are more susceptible to respiratory infections is that hot cigarette smoke disables the cilia, slowing the movement of mucus and bacteria out of the respiratory tract.

Non-specific immune defenses

Non-specific lymphocytes carry out "search and destroy" missions within the body. If these cells encounter a foreign microorganism, they will either engulf the foreign invader or destroy the invader with enzymes. The following are non-specific lymphocytes:

Macrophages are large lymphocytes that engulf foreign cells. Because macrophages ingest other cells, they are also called phagocytes (phagein, to eat + kytos, cell).

Neutrophils are cells that migrate to areas where bacteria have invaded, such as entrances created by cuts in the skin.

Neutrophils digest microorganisms and release microorganism-killing enzymes. Neutrophils die quickly; pus is an accumulation of dead neutrophils.

Natural killer cells kill body cells infected with viruses by punching a hole in the cell membrane, causing the cell to lyse, or break apart.

Fever response is a non-specific response to bacterial or viral invasion. The body responds by increasing its internal temperature, creating conditions that are hostile to the growth of the virus or bacteria.

The inflammatory response is an immune response confined to a small area. When a finger is cut, the area becomes inflamed--red, swollen, and warm. These signs are evidence of the inflammatory response. Injured tissues send out signals to immune system cells, which quickly migrate to the injured area. These immune cells perform different functions: some engulf bacteria, others release bacteria-killing chemicals. Other immune cells release a substance called histamine, which causes blood vessels to become wider (dilate), thus increasing blood flow to the area. All of these activities promote healing in the injured tissue.

When the body's immune system reacts to pollen (a harmless substance) as if it were a bacterium, an immune response is prompted. Histamine is released which dilates blood vessels, causes large amounts of mucus to be produced, and stimulates the release of tears. To combat these reactions, many people take antihistamines, drugs that deactivate histamine.