MIKE SPRINGER/Staff photoJames Brown and his wife Deirdre, left, and their friend Annie Maniates, who plans to donate a kidney to him, at the Causeway Restaurant in Gloucester.

ROCKPORT, Mass. — Deirdre Brown says she and Annie Maniates, her best friend since their days at Rockport High on the Massachusetts coast, have a relationship more like sisters than friends.

“We’re like family,” she says. “We help take care of each others’ kids, the kids play together and all get along — it’s great.”

Soon, however, Maniates will become a part of Deirdre and James Brown’s family in a way that none could have envisioned even a few months ago.

Come Sept. 21, James Brown, 36, will undergo a kidney transplant at the Lahey Hospital & Medical Center in Burlington. And his wife’s longtime best friend will be the live donor.

“I’m so absolutely grateful,” James said.

The 39-year-old Maniates, who served as matron of honor at the Browns’ 2006 wedding, said that as soon as she learned of James’ need this past spring, she never hesitated to try to help.

After being diagnosed with and treated for congestive heart failure, James’ doctors noticed loss of kidney function that didn’t ease as initially expected. He began undergoing dialysis as his doctors recommended a kidney transplant and — given James’ age and overall good health — advised he seek a live donor.

That was when a handful of family members and friends offered to be tested as potential donors, and Maniates proved to be the best match — nearly an ideal one.

“It was incredible,” she said. “They said we couldn’t have been closer unless we were twins.”

According to the Columbia University Department of Surgery, donor compatibility is established through blood tests that look for matching blood types and antigens. Siblings have a 25 percent chance of being an “exact match” for a living donor and a 50 percent chance of being a “half-match.” (The likelihood of a person finding a compatible match in a non-relative varies widely, depending on one’s blood type and the blood’s antibody levels.)

‘We’re so lucky’

Maniates said she’s “not nervous at all” as James’ transplant date approaches, even though her surgery as donor will actually be more invasive than his.

“Don’t get me wrong, I’ll be glad when it’s over, but I’m not really worried,” Maniates said.

All of the friends know they’re facing some difficult times ahead. For one thing, James Brown and Maniates are both expected to be out of work for up to six weeks; Deirdre Brown says she will also be out for two or more weeks as James undergoes his transplant, is taken off dialysis and begins what she says is his “new life.”

“I know — we both know — we are just so, so lucky,” James Brown says, expressing Deirdre’s sentiments as well. “It’s unbelievable.”

Ray Lamont is a staff writer for the Gloucester (Massachusetts) Daily Times.