Last week, Warren Sallach, a road maintenance worker from Brenham, Tex., called Matt Hoffman, a senior defensive end at Rowan University in Glassboro, N.J. They had never met, and Sallach froze when Hoffman answered.

“You don’t know what to say the first time you talk to somebody that saved your life,” Sallach said. “Just a thank you is nowhere near enough.”

In November 2009, Sallach received a blood stem cell transplant to treat non-Hodgkin’s lymphoma in San Antonio. He has since learned that the anonymous donor was Hoffman, who gave up the final football game of his junior season to undergo the donation procedure in Philadelphia.

To protect the confidentiality of recipients and donors, the National Marrow Donor Program requires them to remain anonymous to one another for at least a year after a transplant. They can make contact only if both parties consent.