From a recent article in Real Simple magazine...
"Stay-Well Strategies"

#6 on the list is Pet something fluffy.
"Strange but true: A study conducted at Wilkes University found that stroking a dog for 18 minutes led to a significant spike in secretory immunoglobulin A (IgA), which helps protect the body against germs trying to enter it.  Again, chalk this up to the power of pleasure, says Charnetski, which triggers the release of chemicals in the brain that enhance immune function.  That's why it pays to pet your dog or cat (or your neighbor's) as often as possible.  If you're not an animal lover, don't worry.  A study conducted at the University of Zurich revealed that touching a loved one in an affectionate way-rubbing each other's shoulders or neck, say-has a similar effect.  A simple act like that can help lower levels of the stress hormone cortisol, which can hamper white blood cell function."