The Body's Defense

The body's defense is better known as the immune or lymphatic system. The immune system is a network of cells, tissues, and organs that work together to defend the body against attacks by invaders. These invaders are primarily microbes or tiny organisms such as bacteria, parasites, and fungi that can cause infections. The human body has ideal conditions for many microbes. It is the immune system's job to keep them out or, if they make their way inside the body, to seek out and destroy them.

The first line of defense of your body against disease is your skin, cilia in your respiratory tract, tears, and saliva.

The second line of defense consists of your non-specific defense cells. If pathogens get past the first line, the diameter of the blood vessels increases so that leukocytes can get to the area of infection and histamines are produced. Macrophages

engulf pathogens. In the meantime, the production of white blood cells is increased and the body may induce a fever to stop the pathogens from reproducing. The killer cells are responsible for taking care of the body's infected cells.

The third line of defense is the immune system. The organs of the immune system are positioned throughout the body. Bone marrow, the soft tissue in the hollow center of bones, is the ultimate source of all blood cells. The thymus is a lymphoid organ that lies behind the breastbone. The function of the thymus is to change lymphocytes into T-cells. The spleen is a flattened organ at the upper left of the abdomen. The spleen serves as a meeting ground where immune defenses confront antigens. The lymph nodes function as an immunologic filter for the bodily fluid known as lymph. Antigens are filtered out of the lymph in the lymph node before returning the lymph to the circulation. Other clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract, airways, and lungs. These tissues include the tonsils, adenoids, and appendix.

The immune system stockpiles a huge arsenal of cells including lymphocytes, phagocytes, and granulocytes. The major function of B lymphocytes is the production of antibodies in response to foreign proteins of bacteria, viruses, and tumor cells. Antibodies are specialized proteins that specifically recognize and bind to one particular protein.

Antibody production and binding to a foreign substance or antigen, often is critical as a means of signaling other cells to engulf, kill or remove that substance from the body. Phagocytes are large white cells that can swallow and digest microbes and other foreign particles. Some immune cells take on all intruders, whereas others are trained on highly specific targets. To work effectively, most immune cells need the cooperation of their comrades. Sometimes immune cells communicate by direct physical contact, and sometimes they communicate releasing chemical messengers. Macrophages play many roles. As scavengers, they rid the body of worn-out cells and other debris and they produce a variety of chemical signals which are vital to the immune response. Granulocytes are another kind of immune cell. They contain granules filled with chemicals that allow the granulocytes to destroy microorganisms. Some of these chemicals, such as histamine, also contribute to inflammation and allergy. Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. Some stimulate nearby B cells to produce antibodies, others call in microbe-gobbling cells called phagocytes, and still others activate other T cells.

The immune system stores just a few of each kind of the different cells needed to recognize millions of possible enemies. When an antigen first appears, the few immune cells that can respond to it multiply into a full-scale army of cells. After their job is done, the immune cells fade away, leaving sentries behind to watch for future attacks.

The interactive object above is adapted from the National Institute of Allergy and Infectious Diseases.

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