GUILDERLAND — Five-year-old Kati Harrington says she has “special blood.”

She also has a special brother.

On Sept. 19, her brother, Michael Ardito, will swim a mile across Boston Harbor to raise funds to fight a rare condition that Kati suffers from — hereditary angioedema, called HAE.

Last year, Michael and a friend swam in the harbor’s 62-degree water — heart-stopping cold, he said — to raise $8,000 to fight the disease. This year, he will be joined by three other Guilderland High School students who, like himself, are varsity swimmers on the Guilderville team, made up of both Guilderland and Voorheesville students.

Those who will be swimming with Michael are Alex Cresanti and Jordyn Sommo — both sophomores like Michael — and junior Zach Connolly.

But Michael and his friends are raising more than money. They are also raising awareness about a disease that affects only one in every 50,000 people.

“The most important thing is awareness because this disease is so rare,” said Michael. “That’s why there isn’t more funding to find medication and a cure. When I say the name, no one knows what it means. Most doctors don’t even know.”

He also said, “It can be fatal and the swelling can be triggered by the smallest thing...like vibrations on a lawnmower.”

HAE patients have a defect in the gene that controls a blood protein called C1 Inhibitor, according to the Hereditary Angioedema Association, which explains the condition this way: “Normal C1-Inhibitor helps to regulate the complex biochemical interactions of blood-based systems involved in disease fighting, inflammatory response and coagulation. Because defective C1-Inhibitor does not adequately perform its regulatory function, a biochemical imbalance can occur and produce unwanted peptides that induce the capillaries to release fluids into surrounding tissue.”

This causes swelling in various body parts, including the hands, feet, face, and airway. Patients often have bouts of excruciating abdominal pain, nausea, and vomiting that is caused by swelling in the intestinal wall. Airway swelling is particularly dangerous and can lead to death by asphyxiation.

Nicole Harrington knows this from her own experience. Soon after she married John Harrington, he suffered from an episode of swelling that closed his throat. “He was in a coma for four or five days,” she said. “During that stay, they figured it out.”

It took years for Mr. Harrington to get the proper diagnosis, which is typical of the disease. “He had multiple episodes of severe swelling his whole life,” said Mrs. Harington. “They thought it was an allergic reaction.”

She sited cases where exploratory surgery had been performed or an appendix removed, both because doctors hadn’t recognized the HAE symptom of swelling.

Children of an HAE patient have a 50 percent chance of having the condition themselves. Kati was tested at Boston Children’s Hospital just before her third birthday, Mrs. Harrington said. “She tested positive.”

Kati had two HAE episodes this year — one was in her arm and the other in her leg, her mother said, describing the swelling as “pretty painful.”

“We’d lie on the couch with her a couple of days and then she’s right back at it,” said Mrs. Harrington, adding, “She’s one tough cookie.”

There is no cure for HAE but recently a prophylactic medication has been developed; it was approved by the Federal Drug Administration in late 2008 for use by adults but has not yet gotten FDA approval for use by children. “It cost a lot of money — $48,000 a month,” said Michael.

It’s not only expensive, said Mrs. Harrington, but it’s infused twice a week through a port — a small device implanted beneath the skin, attached to a catheter that goes to a blood vessel so a needle can be inserted to administer drugs.

“It helps him control symptoms,” said Mrs. Harrington of her husband, “but he still gets painful, sometimes life-threatening break-through swelling.”

“Selfless act”

Kati is excited that her brother and his friends will be swimming for her on Sept. 19.  “I’ve dragged her to every swim meet he’s had since she was born,” said Mrs. Harrington.

Michael talked his mother into joining the swim this year. Kati is a little worried about her mother taking the plunge. “She doesn’t see me in the water,” said Mrs. Harrington.

In her youth, Mrs. Harrington, a Voorheesville graduate, was coached by Vaclav Sotola; he now coaches the varsity boys’ Guilderville swim team.

“I’m incredibly nervous,” said Mrs. Harrington of swimming in the Boston Sharkfest Swim 2015. “I’ve been training for it.”

So why is she doing it?

“It’s hard for me to ask for help,” said Mrs. Harrington, “and not do anything in return. You can only ride on your son’s coattails for so long.”

She has instructed the other Guilderland swimmers to wait at the finish line for her. “These kids are really incredible,” she said of Michael and his friends. “The fact that they are willing to swim across Boston Harbor in very cold waters at 9 a.m. on a Saturday morning to help their friend’s sister is amazing to me. This is a completely selfless act on their part.”

The race

Michael has been swimming competitively for eight years. He loves the sport. “The most satisfying thing is seeing your growth through a season,” he said.

Right now, he’s “into marine biology” and thinks he might like to study that and swim in college.

Although he is used to swimming four miles in practice, that’s in a calm pool with well-marked lanes.

Last year’s Sharkfest swim — 1,500 meters or about a mile — was a shock to his system.

“It was very cold. When you first jump in, you have to let your heart rate adjust.”

But then, he said, once his body adjusted, “The cold water becomes nice, helping you not to overheat.”

The swimmers came in three or four waves of 30 to 50 people each, Michael said. “Once you get out half-way, people branch out so you don’t hit each other.”

Last year, he and his friend, Luke Tanner, figured they’d stick together during the swim, but that turned out to be impossible.

“It’s another whole world out there in the open water,” he said. “We went in at once and we tried to stick together.” It may have looked like they did because, Michael said, “We finished at the exact same time.”

But, he went on, “In reality, we had no idea where the other one was.”

Michael concluded, “It’s definitely a challenging race; it’s a unique thing.”

Kati is involved in a race, too, a race with life-or-death consequences.

As Mrs. Harrington put it, “Typically, HAE gets worse with age, so it kind of feels like a race against time for Kati.”

A month ago, Mrs. Harrington said, a 28-year-old woman died of a throat swell in her sleep.

“We are just not where we need to be yet...and unfortunately, I think Mike actually grasps that...He is wise well beyond his years.”

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The Guilderland High School swimmers have three corporate sponsors for the Sharkfest Swim — VENT Fitness, CM Fox, and Serendipity Day Care. Individuals may also make donations through http://bit.ly/1EOJ07p.