New, genetically engineered antibodies can help prevent the body from rejecting transplanted organs, improving the body's acceptance of kidneys more than 40 percent. But the antibodies can be used only once.

After that one use, the body fights off the antibodies, which are designed to temporarily immobilize part of the immune system.

Researchers at 11 medical centers across the country tested the antibody, called OKT3, on 123 transplant patients. After one year, 68 percent of the transplanted kidneys were still working, compared with 48 percent in patients treated with conventional steroid drugs alone to suppress the immune system, according to results published in The New England Journal of Medicine.

OKT3 shuts down the body's T-cells, which are instrumental in the immune system's attack on invaders. This attack is often turned against new organs, destroying them.

But because the body fights the antibody off after the seven- to 14-day treatment, timing of OKT3 use is crucial to success, according to Dr. James Burdick of Baltimore's Johns Hopkins University, one of the test centers.

"Instead of using it early to overcome the initial phases of rejection," Burdick said, "it may be best to hold it in reserve" and use conventional methods. "Then, if the patient has not responded to these drugs, you can use your 'magic wish.'"