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Mind

Crowded Rat Cages: The Silver Lining

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Stress gets blamed for count-less afflictions--from headaches to heart disease. People think they’re more likely to be felled by the flu, say, when they’re under a lot of stress. Yet although a lot of stress over a long time can indeed wear us down, moderate stress may actually spur our body to more effectively battle invaders. So says neuroendocrinologist Firdaus Dhabhar, a doctoral student at Rockefeller University in New York. He recently found that stressed rats have a stronger, more immediate immune response than unstressed animals.

Dhabhar and his colleagues first rubbed a mild chemical irritant onto the skin of ten rats. The researchers then placed five of the rats in a ventilated but cramped seven-inch-long Plexiglas tube for two hours. The five other rats stayed in more spacious cages. When the researchers took blood samples from both groups, they found that the number of T cells and other immune cells had plunged in the rats kept in tubes, while in the other rats it was unchanged.

Conventionally, researchers interpret a low T cell count in the blood as evidence of a weakened immune system. Dhabhar, though, suspected that stressed animals might be strategically redistributing their immune cells to defend against a possible invasion. For the most part, if there is going to be trouble, it will occur at major defense barriers like the skin, where the body interfaces with the outside world, says Dhabhar.

To see whether immune cells might rally to trouble spots, Dhabhar applied the same chemical irritant to the ears of both groups of rats. The irritated area swelled more quickly and three times as large in the confined rats--a sign that immune cells were flooding the area, causing blood vessels to expand to let more immune cells through. Skin samples confirmed that the confined rats had more T cells in the irritated area, despite the lower overall levels in their blood.

The confined rats experience a moderate level of stress, not unlike what people may endure in a packed subway. In both cases, the stress of overcrowding raises the heart rate and increases the animal’s readiness to respond to threats--including pathogens, Dhabhar believes. Just as the fight or flight response pumps adrenaline through our bodies, moderate stress may put the immune system on alert by sending immune cells to potential hot spots. The overwhelming assumption seems to be that stress has a negative effect on the immune system, he says. We’re saying that a moderate amount of stress could lead to this immuno-enhancing effect.

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