Stress reduction may bode ill for some kids.

Some children may experience biological reactions to stress that, under consistently threatening conditions in their social worlds, render them particularly vulnerable to various respiratory ailments, such as nasal congestion, sore throats, and ear infections. However, the same "biologically reactive" youngsters gain protection against such illnesses when they live in nurturing, low-stress surroundings.

That, at least, is the suggestion of a pair of studies in the September-October Psychosomatic Medicine. If future research confirms the findings, efforts to prevent various childhood illnesses may need to focus on kids who show a biological sensitivity to stress.

"Only a subset of individuals may be susceptible to the health-altering effects of stressors and adversity," contend W. Thomas Boyce, a pediatrician at the University of California, San Francisco, and his colleagues.

Boyce's group first conducted a 6-month study of 77 boys and 60 girls, all 3 to 5 years old and living in middle-class families. Every 2 weeks, the researchers rated stress encountered by children at the four child-care centers they attended. Ratings drew on teacher reports of incidents such as a child's getting snubbed by peers, as well as on the ratio of teachers to children and other indicators of child-care quality.

Children classed as biologically reactive displayed large jumps in heart rate and blood pressure on a series of laboratory tasks, such as recalling lists of numbers and identifying incomplete pictures of objects.

A nurse examined the kids each week for symptoms of respiratory illness.

In a second study, the scientists tracked respiratory illnesses in 50 boys and 49 girls, all 5 years old, for 4 months after their entry into kindergarten. Each youngster gave a blood sample for analysis 1 week before and 1 week after kindergarten started. Those treated as biologically reactive showed large changes, either up or down, in the number of several types of immune cells and in the ability of immune cells to proliferate when stimulated chemically.

Psychological stress at the start of kindergarten was assessed through parents' reports of events in the past year, such as divorce or the birth of a sibling, as well as chronic areas of family discord.

In both studies, biologically reactive children experienced modest, but statistically significant, rises in respiratory illness when they faced stressful child-care or family situations. Youngsters exhibiting minimal biological responses to experimental tasks showed no increase in respiratory ailments associated with stress.

Most surprisingly, biologically reactive children who encountered little child-care or school stress suffered fewer infections than all other children.

Exaggerated biological reactions to stress may reflect "a heightened sensitivity to the character of the social world," the scientists propose. Thus, this bodily disposition boosts vulnerability to illness in adverse situations and fosters resistance to illness in supportive surroundings, they theorize.

The new data address unresolved questions about how stress may trigger illness in some individuals but not others, contend Sheldon Cohen of Carnegie Mellon University in Pittsburgh and Stephen B. Manuck of the University of Pittsburgh in an accompanying comment. But the results are preliminary and the theory advanced by Boyce's team remains speculative, the two psychologists contend.

No known physiological process can account for the lower occurrence of illness in biologically reactive kids under low stress, they note. Moreover, it is unclear whether individual immune reactions to stress are as consistent as those of heart rate and blood pressure, Cohen and Manuck add.