The first time George Flocco wrote to Cynthia London to thank her for her son's heart, she didn't reply. Still, he kept trying.

"Every year, for five years, I'd write, me and my wife," Flocco recalled. "Finally, she answered."

London needed the time to process how she lost her 22-year-old son before she could answer Flocco and, later, meet the Bucks County man and his family.

Sipho Themba, a Kutztown State University student and aspiring secondary schoolteacher, "wanted to help children so they wouldn't be lost to the streets," his mother said. He died in what his mother describes as a senseless shooting.

"I had to feel better about how Sipho passed," London explained. "I didn't want to see anybody. . . . I didn't have to know 'my' recipient. I think they [the Floccos] were more curious about us than we were about them."

But eventually, London would become such a passionate and public advocate for organ donation that she now calls herself "the voice of those who are waiting."

She tells her story on the Gift of Life Donor Program website and makes public appearances for the cause. And she is quick to point out that the name "Sipho Themba" is "gift of hope" in the South African language Xhosa.

[Read more: Gift of Life brings donor families and recipients together]

Many details of Sipho's death in the winter of 1997 are unknown, his mother says. How Flocco came close to death is clear.

A salesman for a trucking company, Flocco had suffered a major heart attack the winter of 1996, at age 47. Within months, he was so ill that he was on the top of the transplant list, with perhaps weeks to live.

Right around that time, London, who lives in the Mantua section of West Philadelphia, became familiar with organ and tissue transplantation through a publicity campaign by basketball great Michael Jordan.

"And three months later we were faced with that," she said. "It was so crazy."

When Sipho was pronounced brain-dead, his mother did not hesitate. Her son's heart was transplanted to Flocco at Hahnemann University Hospital. Other organs were transplanted to five more people.

Flocco is now 74, retired and leading a full life. He frequently volunteers at Family House, a lodging facility for transplant candidates, recipients, family members, and living donors for Gift of Life.

His family has occasionally done things with Sipho's. Sometimes he'll hear a remark about having an African American's heart, and he has a ready answer:

"I tell them the heart knows no color, no religion. The only thing a heart knows is red blood."

And as George Flocco has found another life, Cynthia London has found some peace through the donor process.

"We let Sipho go home to the Lord," she said, "but we gave hope to six other people."