The 1984 Nobel Prize in medicine was awarded yesterday to three Europeans whose studies pioneered modern understanding of the body's immune system and paved the way for a promising array of practical applications across all areas of medicine, from AIDS detection to cancer treatment.

Stockholm's Karolinska Institute honored Niels K. Jerne, 72, a London-born Dane who has worked in Switzerland, as the "leading theoretician in immunology during the past 30 years."

He will share the $193,000 prize equally with Georges J.F. Koehler, 38, a West German scientist, and Cesar Milstein, 57, an Argentine-born British researcher, who were cited for discovery of a new technology for manipulating antibodies that "represents one of the most important methodological advances in biomedicine during the 1970s."

In less than a decade, the production of so-called "monoclonal antibodies" has "opened up completely new fields for theoretical and applied biomedical research and allows precise diagnosis and also treatment of disease," said the Karolinska committee that selected the winners.

It compared the importance of this new medical tool to the development in the 1970s of genetic engineering technology, whose developers have already been honored with several Nobel prizes.

Antibodies are a key element in the body's defense system that seek out and destroy foreign invaders. In 1975, Koehler and Milstein announced that they had improved upon nature by developing a new laboratory technique for producing large quantities of antibodies that could be targeted to perform specific tasks.

Although the award went to three foreign researchers -- U.S. scientists have dominated this top prize in years past -- the technology is widely used in this country in research and increasingly is being translated into commercial tests. While few patients are aware of it, since 1981 the Food and Drug Administration already has approved nearly 60 applications using monoclonal antibodies to detect everything from pregnancy to venereal disease.

In addition to providing a new laboratory approach for quickly measuring tiny amounts of difficult to find viruses or toxins, it also makes it possible to distinguish differences in the body's own cells, such as white blood cell changes that occur with AIDS, the deadly acquired immune deficiency syndrome.

In addition, the technology presents an opportunity for targeting human therapy. In particular, it is touted as a possible means of not only finding cancer cells early in the disease process but of getting powerful cancer drugs to cancer cells, killing them directly without affecting normal cells.

The basic technique of producing monoclonal antibodies involves the creation of hybrid cells, called hybridomas, by fusing cancer cells with antibody-producing white cells taken from animals whose immune systems have been stimulated by a given organism or toxin. Because the cancer cells can grow rapidly, it provides a means of growing large quantities of antibodies against the selected target.

Several U.S. researchers hailed the selection of this year's winners.

"It is very well deserved," said Dr. Solomon Snyder, a Johns Hopkins University scientist who recently announced a new technique for improving upon the original Koehler-Milstein discovery. "I can't imagine many discoveries that have as great an impact on the advance of basic science and practical benefits to mankind."

Snyder said the applications of the monoclonal antibody technique thus far represented "only the tip of the iceberg."

Dr. Jeffrey Schlom, chief of the laboratory of tumor immunology and biology at the National Cancer Institute here, said the monoclonal antibody work represented a "major breakthrough" that had given scientists a practical way of "finding the needle in the haystack" in the complex field of immunology.

He said that the technique was now being used in several medical centers to monitor the course of gastric, colon and ovarian cancers.

Milstein, who works in Cambridge, England, at the Medical Research Council laboratory of molecular biology, called the award announcement "a bit of a bomb. I need a bit of fresh air," United Press International reported.

Koehler and Jerne both have worked in recent years at the Institute for Immunology in Basel, Switzerland. UPI said Koehler reacted yesterday with surprise: "Are you sure? This is fantastic, it's unbelievable," he said. "I've got to sit down, and then I'll call my wife." Jerne, who now lives in France, said, "It makes me very happy, of course."

Jerne was credited by the Nobel committee with three "visionary" theories since 1955 that help explain the diversity of the immune system in fighting infection from foreign organisms.