August 27 (HealthDay News) -- The U.S. Food and Drug Administration has approved AlloMap, a non-invasive test that helps doctors identify heart transplant recipients who are rejecting the new organ.

The test monitors the genetic activity of a transplant recipient's white blood cells, the immune system's primary defense against viruses, bacteria and other germs, the FDA said in a news release.

Rejection occurs when a transplant recipient's body begins attacking the new organ as a foreign entity. A successful transplant is achieved when the body's immune response is suppressed enough to accept the new organ but is strong enough to continue warding off infection, the agency said.

Half of all rejections occur in the first six weeks after transplant, and about one-quarter of recipients have evidence of rejection within the first year, according to government data cited by the agency.

AlloMap is produced by Brisbane, Calif.-based XDx Inc.