Millions of people around the world are alive today because someone, somewhere donated their blood.

Today these countless unknowns and the lives they have changed are being celebrated the world over as part of World Blood Donor Day today.

Among the world's blood donors are volunteer crew members onboard Mercy Ship's hospital ship, Africa Mercy.

The Africa Mercy provides free life-saving and life-changing medical treatment to locals wherever it docks, and this often requires blood transfusions to make the procedures possible.

The lack of space on the ship means that blood cannot be stockpiled but with a crew made up entirely of volunteers, there is never a shortage of willing blood donors whenever a call goes out.

What is special in this case is that those who have been saved through the blood transfused from a crew member of the Africa Mercy have the chance to meet their donors face to face.

“We call it a walking blood bank,” says Claudia Juarez, a senior lab technologist on the Africa Mercy.

“When crew members arrive, they have the option to become blood donors. If they agree, they are screened and know that they could be asked to donate whenever the need for blood arises."

So far this year, 209 crew members on the Africa Mercy have signed up to be blood donors and 162 of those cross-matched with 95 patients who may need blood for their surgical procedures. A total of 37 units of blood have been donated.

One lucky recipient was 23-year-old Harding Sesay of Guinea, West Africa. He arrived at the Africa Mercy with a boil on his nose that, without access to a doctor or medical treatment, had grown so large that it had blocked the vision in his right eye.

He was lucky enough to have the tumour removed by doctors on the Africa Mercy whilst it was docked in Sierra Leone.

His donor was ward nurse Corina Buth, who had prepared Sesay for surgery and walked him into the operating room, where she stayed to witness the operation to remove his tumour.

Sesay suffered massive blood loss during the surgery and Buth was called upon to donate a unit of blood. No sooner had she donated her blood and it was being transfused into Sesay.

As he recovered the next day, he was unable to speak, but Buth understood his feeling.

“His eyes told the whole story. I knew he was thankful,” she said.

Prior to his surgery, Sesay had spent seven years hiding from the world because of the tumour on his face. Now he is thankful that he can enjoy life once more.

“For so many years I suffered. Now, I am happy, and I’m full of joy. Thanks to God for what Mercy Ships has done for me.”