In 2006, a one-year-old New Yorker named Matthew Welling, born with a potentially fatal disease, received a bone marrow transplant from a 49-year-old man from Boston.
The Boston man despised the New York Yankees as much as Matthew's father loved them.
Dad hated the Boston Red Sox with the same intensity that the donor worshipped them.
A year later, Mike Welling took his young son to meet Steve Karas, the man who saved his life, in his Boston home. "We were entering enemy territory," Mike said. He and Karas became friends "once I got past the fact that Steve was a Red Sox fan".
The friendship has endured despite Steve taunting Mike with: "Matt now has Red Sox blood in him."
There is no rivalry in American professional sports as fierce as the Red Sox versus the Yankees.
They face off 18 times each regular season to sellout crowds. Their games are an automatic choice for national television because they draw huge ratings.
In New York, they mock the Red Sox Nation for their name - nothing "national" about your following, they say - and for trying to do it the Yankee way, but with far less to show for it.
In Boston, otherwise intelligent people attribute the World Series title drought that ended in 2004 to the Curse of the Bambino - the trade of Babe Ruth from the Red Sox to the Yanks more than 80 years ago.
A construction worker with Red Sox leanings planted a Boston jersey in the foundations of the new Yankee Stadium two years ago as an intended curse. The Yanks took the time and trouble to remove the shirt by digging into two feet of concrete.
And we thought only big-city gangsters met such a fate.
The franchises pay mountains of money on players, sometimes recklessly. Last season was typical - US$206 million (Dh756.6m) for the Yankees payroll, $163m for Boston, well ahead of everyone else.
When the chintzy Tampa Bay Rays and their 21st-rated payroll of $72m won the National League East, you knew the roster architects in the Big Apple and Beantown would spring into action this off-season.
The Yankees held on to superstars Derek Jeter and Mariano Rivera by extending their contracts. They also engaged in a full-bore bidding war for Cliff Lee, the lights-out pitcher.
This is how the Yankees do business, courting top players while doling out deals to loved ones like the declining Jeter, who got three additional years.
Meanwhile, the Red Sox were playing their own game. They made a token bid for Rivera just to grab a headline, knowing he will someday be buried in pinstripes. They also feigned interest in Lee.
Then, they struck.
First, they pried Adrian Gonzalez, the first baseman, from the pauper San Diego Padres for the rock-bottom price of three prospects. Second, they captured Carl Crawford, the Rays free agent outfielder who had soared beyond Tampa Bay's price range.
Old-timers refer to off-season happenings as the Hot Stove League, and these two proud clubs compete as if there is a pennant at stake.
If standings were kept, Boston would be in the lead.
Both of their newcomers are a shade under 30, meaning their expiration playing date is not near.
Crawford, 29, came at a cost of $142m, the second-most ever for an outfielder, for seven years. The team are trying to similarly lock up Gonzalez, 28, whose deal runs out after this season.
These two could be teammates through the next three paint jobs of Fenway Park's Green Monster wall.
Crawford hits better than .300, runs like his cleats are on fire and has recently found a power source.
With Gonzalez, it is safe to write in 30 home runs and 100 RBIs on his season stat line. He is as consistent as frigid winters in Chicago.
The buzz in Boston is not limited to their new offensive potential. Both acquisitions are Gold Glove winners. The reformed Red Sox, who have never made defence a priority, now are regarded among the major leagues' masters with the mitts.
The Yankees, beholden to six graying players with massive contracts until 2013, are like the home owner with three mortgages. They have less room for error on other spending decisions.
The Red Sox, meantime, have the bulk of their line-up under wraps for three more seasons.
Of course, the Yankees may work more chequebook magic with other players. The most intense rivalry of all never rests.
Little Matthew Welling, who is doing well four years after the bone marrow donation, might be old enough to talk some serious New York-Boston trash with Steve Karas, the man who extended his life.
And Karas might feel comfortable enough to dish it right back.
It is Red Sox versus Yankees, where pride is thicker than blood, even a donor's.
Cliff Lee returns to Phillies, s14