BALTIMORE, JUNE 11 -- William Cooper, a 37-year-old hospital therapist, radiates energy and robustness. He has a quick smile and lively eyes.

As he stood in his crisply tailored suit at a Red Cross news conference here today, nothing betrayed the fact that he is suffering from sickle cell anemia and needs transfusions of an extremely rare kind of blood to keep the sometimes excruciating pain in his body from overcoming him.

Appearing with Cooper, officials of the Chesapeake Regional Red Cross Blood Services launched an appeal for help from black donors, believed to be virtually the only carriers of the unusual blood that Cooper has.

Despite a national search through the Red Cross's computerized Rare Donor Registry, no donor with Cooper's combination of blood factors has been found.

The Red Cross urged blacks, who historically have been less active in blood donation programs than whites, to undergo screening for a range of rare blood factor combinations that could be helpful not only to Cooper but also to all sickle cell sufferers, most of whom are black.

"If I can help educate my people to understand that their blood can be special, then I will gladly speak out," said Cooper, who has had sickle cell anemia since early childhood. Sickle cell patients require frequent blood transfusions.

In recent transfusions, Cooper acquired an uncommon combination of new "markers," or antibodies in his blood, including the extremely rare Cromer antibody. Red Cross officials estimated the chance of finding Cooper's exact combination in another person is 1 in 100,000.

Nevertheless, they say they are hopeful of finding just that combination among the thousands of potential black donors in the Maryland area.

One of every 500 black Americans has sickle cell anemia in varying forms of severity and with varying combinations of antibodies, according to the Red Cross.

As sickle cell patients develop new antibodies, it becomes increasingly difficult to match them with donors with the same combinations.

Sickle cell anemia is a genetic disease occurring most frequently among blacks in the United States. It gets its name from the fact that hemoglobin in the red blood cells crystalizes into rod-like forms that cause the usually flexible cells to become rigid in crescent moon or sickle shapes. These rigid cells cluster in the smaller blood vessels, blocking the flow of blood and oxygen to vital organs of the body. Lack of immediate treatment can cause death.

Cooper said he suffers periodic severe pain when his blood flow is blocked. Hydration and other efforts to enlarge the blood vessels bring limited relief, he said, but blood transfusions are much more effective.

"I can't stress too much what it's like to lay there and not have the blood available," he said. " . . . Blood is the lifeline."

He has been hospitalized periodically in recent months but has managed to continue work as a respiratory therapist at Mercy Hospital in downtown Baltimore.

Red Cross officials, noting that only 5 percent of the general population gives blood, said they are not certain why blacks participate even less.

It may be partly because blacks make up a relatively large part of the nation's lower-income population, which itself typically does not participate in blood donation programs, officials said.

Also, the officials said, even among blacks willing to donate blood, a disproportionately large percentage