Dr. John A. Hansen, an immunologist whose pioneering research made bone marrow transplants safer and vastly expanded the pool of potential donors to patients with leukemia and other blood disorders, died on July 31 at his home in Mercer Island, Wash. He was 76.

The cause was pancreatic cancer, his wife, Suzanne Hansen, said.

As a clinical researcher at the Fred Hutchinson Cancer Research Center in Seattle from 1977 until he retired last year, Dr. Hansen had a profound impact on the treatment of leukemia, lymphoma and blood and immune system diseases.

His team in Seattle developed sophisticated methods of determining which combination of chemicals in a patient’s immune system is required to keep his body from rejecting a marrow transplant from a donor — either a sibling or a complete stranger — whose tissue type does not precisely match.

With Drs. Jeffrey McCullough in Minnesota and Herbert Perkins in San Francisco, Dr. Hansen also helped establish a national registry called bethematch.org, which links patients awaiting transplants with potential donors of bone marrow, blood stem cells and a newborn’s umbilical cord blood.