If a patient had a vasectomy, how could the forensic scientists analyze his semen?

Semen and sperm are different. A man who has a vasectomy still produces semen. That spermless semen, which may also be referred to as seminal fluid, is still of interest to forensic scientists. That’s because it could be used to identify a suspect in a crime or contribute other information to an investigation.

So how did his donor’s DNA end up in his spermless semen?

To say definitively what happened in Mr. Long’s case would require more research. But several experts in bone marrow transplants said that the forensic scientists’ findings made sense from a medical standpoint and that the answer involved white blood cells.

The two most common types of cells in semen are sperm cells and white blood cells, said Dr. Mehrdad Abedi, a bone marrow transplant specialist at the Comprehensive Cancer Center at the University of California, Davis, who treated Mr. Long but was not involved in the experiment.

“Because he had a vasectomy,” he said, “all that’s left is the white cells.”

Dr. Elias Zambidis, a stem cell investigator at the Institute for Cell Engineering at Johns Hopkins University School of Medicine, agreed that the analysts were detecting the donor’s DNA in white blood cells.

Semen contains a number of different types of white blood cells, including macrophages, neutrophils and lymphocytes — all of which support the immune system. “These can come from the urinary tract or the prostate, and will be present even in a vasectomized man,” Dr. Zambidis said. “Since all white blood cells come from the bone marrow donor after a bone marrow transplant, it will not be a surprise that ejaculate containing white blood cells will also be of the donor.”