Adapted by Thyroid Guide, Mary J. Shomon
Return to Index for "Understanding the Immune System"
Mounting an Immune Response
Infections remain the most common cause of human disease. Produced by bacteria, viruses, parasites and fungi, infections may range from relatively mild respiratory illnesses such as the common cold, to debilitating conditions like chronic hepatitis, to life-threatening diseases such as AIDS and meningitis.

To fend off the threatening horde, the body as devised astonishingly intricate defenses. Microbes attempting to enter the body must first find a chink in the body's external protection. The skin and the mucous membranes that line the body's portals not only pose a physical barrier, they are also rich in scavenger cells and IgA antibodies.
Next, invaders must elude a series of nonspecific defenses-those cells and substances equipped to tackle infectious agents without regard for their antigenic peculiarities. Many potential infections are cut short when microbes are intercepted by patrolling scavenger cells or disabled by complement or other enzymes or chemicals. Virus-infected cells, for instance, secrete interferon, a chemical that rouses natural killer cells.
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Updated 2002: This information has been made available from the National Cancer Institute and the National Institute of Allergy and Infectious Diseases.

