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Scientists delving into one of the immune system's deepest mysteries reported Friday they have discovered the genetic trigger that equips the body to hunt down a bewildering array of invading germs.

One expert described the discovery as one of the most important ever made in the young field of immunology, the study of the body's built-in defenses against disease.Researchers from the Whitehead Institute for Biomedical Research said they found a gene that ceaselessly reshuffles the genetic library inside newly made white blood cells. This lets the body use a limited amount of genetic material, or DNA, to concoct an endless assortment of antibodies capable of zeroing in on bacteria and viruses that it has never before encountered.

Their discovery should improve understanding of how the body arms itself against infection.

The gene - dubbed the recombination activating gene, or RAG-1 - was discovered by graduate students David G. Schatz and Marjorie Oettinger. They described their work in today's issue of the journal Cell.

"It's one of the top 10 or 20 discoveries in the history of immunology," commented Dr. Michael Lieber of Stanford University. "It deals with the basic way that our bodies make antibodies."

The researchers believe the gene makes all or part of an enzyme called recombinase, which cuts and splices bits of DNA. These scraps of genetic material are assembled into new genes, which in turn provide the danger-sensing radar for B cells and T cells, the body's principal warriors against invading microbes and cancer.

B cells work by secreting uniquely shaped chemicals called antibodies. Occasionally, an antibody encounters a microbe with a spot on its surface that is a mirror image of the antibody. When this happens, the antibody latches onto this spot, known as an antigen. This marks the invader for destruction by other soldiers in the immune system.