It's been long rumored that folks with tattoos are inherently tougher.  I'm not sure what it is exactly, but even a righteous Hello Kitty tattoo can make you look like one rough hombre.  Recent scientific study has revealed that tattoos do make you tougher, immunologically speaking.

The science breaks down like this:  When you get your first tattoo, your body releases a chemical known as cortisol to deal with the stress.  This, in turn, causes a drop in immunoglobulin A - an antibody that your body uses to fight off disease.  It's the same reason you are more likely to get sick when you are stressed out.  When you get subsequent tattoos (2nd, 3rd, 4th, etc), the study shows that your stress level is significantly lower each time.  Your body needs less and less cortisol, and retains higher levels of immunoglobulin A.  Scientists conducting the study say this "stress-resistance," translates to other stressful events in life (traffic, annoying co-worker, screaming kids, getting a ticket, etc) making you more resilient to the stress generated in those situations as well!

According to the official report:

...the body habituates over time to the tattooing stressor. It is possible that individuals with healthy immune systems heal faster, making them more likely to get multiple tattoos."