Of the two most common yellow jacket species found in the eastern US, Vespula maculifrons -- which tends to live in large underground nests -- is much more likely to cause a systemic reaction than is Vespula germanica, Dr. David B. K. Golden of the Johns Hopkins Asthma and Allergy Center in Baltimore and colleagues found.

This means that if a person with a history of severe reactions to yellow jacket stings gets stung again without reacting severely, the milder sting may simply have come from the milder species -- and doesn't mean the person has outgrown the allergy, Golden explained in an interview with Reuters Health.

"What we hear from people is that 'oh it was a fluke, it will never happen again'," he said. "That's certainly not something you can count on."

A systemic reaction extends beyond the area where a person was stung, and can range from mild responses such as light-headedness to potentially fatal reactions such as swelling that closes off the windpipe or a dramatic drop in blood pressure.

Golden and his team initiated their study to determine why some people react severely to yellow jacket stings and others don't. They tested reactions to yellow jacket venom in 111 healthy volunteers, who received a total of 175 sting challenges over a three-year period, and report the results in the Journal of Allergy and Clinical Immunology.

V. maculifrons was more likely to produce a systemic reaction than V. germanica, the researchers found. And these reactions were more common among people who had experienced such a reaction previously. Among people with a history of systemic reactions to yellow jacket stings, 41 percent had a systemic reaction to V. maculifrons. But among those who had not experienced systemic reactions in the past, just 3 percent exhibited systemic reactions to V. germanica.

The findings show that even if a person has escaped a severe reaction with a yellow jacket sting in the past, he or she is not safe from having one in the future, the researchers conclude.

Anybody who has suffered a severe reaction to a wasp, bee or yellow jacket sting should be evaluated by an allergist, Golden told Reuters Health, because immunization is available for allergies to each type of venom that is 98 percent protective against future severe reactions.

SOURCE: Journal of Allergy and Clinical Immunology