The Immune System
A second line of defense is housed within the body: a finely tuned immune system that recognizes and destroys foreign substances and organisms that enter the body. The immune system can distinguish between the body's own tissues and outside substances called antigens * . This allows cells of the immune army to identify and destroy only those enemy antigens. The ability to identify an antigen also permits the immune system to "remember" antigens the body has been exposed to in the past, so that the body can mount a better and faster immune response the next time any of these antigens appear.
Lymphocytes (LIM-fo-sites), white blood cells that develop in bone marrow * and circulate throughout the body in the lymphatic system * , are a vital part of the immune system. Lymphocytes can be divided into two subgroups: B lymphocytes and T lymphocytes. B lymphocytes (or B cells) produce immunoglobulins (ih-myoo-no-GLAH-byoo-lins), also called antibodies (AN-tih-bah-deez). These protein molecules attach themselves to specific antigens and work with another type of white blood cell, called phagocytes (FAH-go-sites)—scavenger cells that surround and digest infected cells or microorganisms—to destroy the invaders. T lymphocytes (or T cells) help control the immune response and destroy foreign antigens directly.
The activity of B cells and T cells targets specific antigens. This means that each time a new kind of antigen invades the body, the immune system must produce a new round of B cells and T cells, which attack only that antigen. It is estimated that the immune system can create more than 100 million types of antibodies. As B cells and T cells mature, they begin to recognize which tissues belong in the body and which do not. These cells become "memory" cells that remember a particular antigen, so that the next time it appears, the immune response can mobilize quickly. In some cases, people have permanent immunity * to a disease; for example, people who contract chicken pox usually will not have it again—or, if they do, they will have a much more mild case.
The immune system works with amazing complexity. When a B cell encounters a foreign invader, it starts to produce immunoglobulins, or antibodies. Like a key designed to fit only a specific lock, an antibody "locks" onto a single type of antigen like an identifying marker. Once the antibody attaches to an antigen, one class of T cells called helper T cells alerts other white blood cells to head toward the site, while another class called killer T cells begins to destroy the antigen marked by the antibody. At the same time, millions of antibodies swarm through the bloodstream to attach to any more of that type of antigen and mount a larger attack.
The immune system also includes other proteins and chemicals that assist antibodies and T cells in their work. Among them are chemicals that alert phagocytes to the site of the infection. The complement system, a group of proteins that normally float freely in the blood, move toward infections, where they combine to help destroy microorganisms and foreign particles. They do this by changing the surface of bacteria or other microorganisms, causing them to die.
* enzyme (EN-zime) is a protein that helps speed up a chemical reaction In the body.
* antigens (AN-tih-jens) are substances that are recognized as a threat by the body's immune system, which triggers the formation of specific antibodies against the substance.
* bone marrow is the soft tissue inside bones where blood cells are made.
* lymphatic (lim-FAH-tik) system is a system that contains lymph nodes and a network of channels that carry fluid and cells of the immune system through the body.
* immunity (ih-MYOON-uh-tee) is the condition of being protected against an infectious disease. Immunity often develops after a germ is introduced to the body. One type of immunity occurs when the body makes special protein molecules called antibodies to fight the disease-causing germ. The next time that germ enters the body, the antibodies quickly attack it, usually preventing the germ from causing disease.
In some instances, people receive antibodies from another person to help their own immunity. This is known as passive immunity. Infants are born with immature immune systems and receive important antibodies
The human body has several lines of defense against infection, which work to prevent germs from invading the body or to destroy them once they find their way in.
from their mothers, both during pregnancy (across the mother's placenta * ) and after birth from breast milk. These antibodies usually disappear within 6 to 12 months, but until then they help protect the infant against a range of infections, including pneumonia * , bronchitis * , influenza * , and ear infection. Doctors also can give people gamma globulin (GAH-muh GLAH-byoo-lin), an antibody preparation that offers temporary immunity to patients who might need this protection. When a person gets an immunization, or vaccine * , the body's immune system learns to recognize that particular bacteria or virus. If, sometime later, the person is exposed to the germ again, the body can fight it off and not come down with the disease. Read more:
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