Unreasonable Searches When Going to Work?

Chico Science asks: "I'm a scientist, not a lawyer, so I'm a little beleaguered by the fact that since 2001-Sep-11, I have been forced to submit to searches on my campus as I enter buildings. I work at the National Institutes of Health in Bethesda, MD, and have been shouldering the burden of increasingly draconian security measures. Most recently, they've instituted a policy of 100% bag/package searches on entering buildings. Initially it didn't bother me, but after having my bag searched on my way to my car (which was also thoroughly inspected) after work, I decided I'm not comfortable subjecting myself to searches of my personal belongings at every turn. I want to know if I have a right to refuse searches? And why should it be considered acceptable for me to relinquish my Fourth Ammendment rights so I can go work on in my lab?" In this climate of increasing security consciousness, how far can vigilance go before it becomes an invasion of our rights?