I've got an allergy to the cold. No, really! But that doesn't mean I spent more time warming the couch in the winter. Once I understood my "allergy," I learned how to dress and what to do to stay with it.

My allergy is officially called Raynaud's phenomenon, a mystery circulatory disorder marked by exaggerated sensitivity to cold. Raynaud's (pronounced RAY-noze) goes beyond the simple nuisance of cold hands and feet many people experience. If someone has Raynaud's, "chills" as warm as 50 or 60 degrees can trigger an attack. An air-conditioned theater in the summer or even reaching into the freezer can cause pain.

This is what happens: Fingers and toes go numb, quickly turning sheet white as all blood flow stops. Sometimes a dramatic line between the palm's rosy flesh and the white of bloodless fingers is marked. As the attack wanes, fingers turn blue as deoxygenated blood sluggishly returns. The end of the attack is even more painful as fingers flush deep red, burn and throb. No wonder I panic if I don't have my gloves.

"Raynaud's is excruciating," says Andy Young, environmental physiologist, formerly with the U.S. Army Research Institute of Environmental Medicine. "The people who have it really suffer."Of the millions of sufferers, three-quarters are women, who usually first experience symptoms between the ages of 15 and 40. Some estimates say as many as 20 percent of all women have some form of Raynaud's.

The syndrome, despite first being described in 1862 by French physician Maurice Raynaud, still leaves medical researchers perplexed why some people can't cope with cold. Normally, skin receptors in the fingers sense cold and send messages that eventually signal blood vessels in the fingers to constrict so more blood can pump to vital internal organs. The system works in waves so fingers are never totally deprived, says Robert Freedman, who has studied Reynaud's for some 20 years.

Freedman says it now appears there is a biochemical abnormality in the natural temperature gauges in the fingers of Raynaud's sufferers. That causes blood vessels to shut down shop completely. Lucky us.