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"We have observed this relationship in animal studies but this is the first time that a connection has been made between aggression and immunity in humans," says Douglas Granger, associate professor of biobehavioral health in Penn State's College of Health and Human Development.

Granger and colleagues believe that men who have been in occasional fights or in trouble with the law have immune systems that may be ready to launch a more rapid and intense response to pathogens than do the immune systems in men who are seldom aggressive.

"Our study suggests that differences in people's aggressive behavior influences how their immune systems are prepared to deal with infections, viruses and bacteria," says Alan Booth, Distinguished Professor of Sociology and Human Development and Family Studies at Penn State.

The researchers studied a sample of 4,415 men, aged 30 to 48 years, who were interviewed to determine their level of aggressive behavior. Subjects then underwent a medical examination to determine their state of health. The researchers also took blood samples from each subject. Those samples were then analyzed for different types of white blood cells or lymphocytes.

"White blood cells are major players in the body's immune system," explains Granger. Out of eight enumerative indicators of immune activity studied, two specialized types of lymphocytes (CD4 cells and B cells) that determine the initiation, magnitude, and duration of specific cellular immune responses were present in high concentrations in the circulation of moderately aggressive men.

Granger believes that the activity of these particular lymphocytes, which includes antibody production and secretion of intercellular signals that turn the immune response on or off, has considerable value for increasing the chances of survival in a pathologically challenging environment.

According to the study, individuals who reported engaging in two aggressive acts were 30 percent more likely to be in the top quartile of CD4 cell numbers than those reporting no aggressive acts. However, increases in aggressive behavior did not convey correspondingly higher odds of being in the top quartile.

Men reported on 12 different acts of aggression ranging all the way from playing hooky twice a year or more to fights involving weapons.

"The strength of the finding is that we controlled for all types of factors that could impact the subjects' immune systems, such as whether the subjects smoked or consumed alcohol, their level of health and their testosterone scores," says Booth. "While testosterone was associated with aggressive behavior, it was not the hormone that accounted for the higher immune cells found among aggressive men."

The study was a joint effort between researchers at Penn State and the University of Nebraska and appears in the current issue of Psychosomatic Medicine, the professional journal for the American Psychosomatic Society.