Leukocytes are always present in the body, but most remain dormant until an immune response is activated by wounding or infection or until the brain identifies a stressful situation. When that happens, the brain releases hormones that set troops of these immune cells into motion. The cells travel to potential battle stations primarily the skin along with the lymph nodes that drain the skin.
"Most immune challenges or wounds involve on the skin or other epithelial linings of the body," Dhabhar said. "If nothing happens immunologically following stress the skin isn't cut or wounded in some other way activated leukocytes usually return to their resting position in a few hours."
Some of the mice in the study were restrained in clear plastic ventilated tubes for two-and-a-half hours. These mice the stressed group could not turn around in the tube, but they could move forward and backward. This restraint created a brief spell of psychological stress, similar to the kind of stress a person anticipating or undergoing a dental or surgical procedure may feel. The other group of mice the non-stressed group remained in their home cages.
Once the stressed mice were removed from the tubes, the researchers implanted tiny sponges underneath the skin on the backs of all of the animals, including the mice in the non-stressed group. These disc-shaped sponges were about the size of a grain of rice.
Sponges were removed from some of the mice six hours after implantation and from the rest of the mice one, two or three days later. The researchers compared the numbers of leukocytes in each sponge once the sponges were removed.