The word 'piacere' in Italian means "nice to meet you." Both here in Italy, as well as at home in the U.S., handshakes are a common way to either greet or say goodbye to someone. Here in Italy they also use kisses on each cheek as a greeting (as well as in many other parts of Europe).
For allergic reactors, greeting people can sometimes to be challenging. There have been numerous times where I have nervously given out my hand to shake, while hoping to not break out into hives. It is hard to awkwardly explain my allergies to someone I am just meeting or don't know well. I don't want to hesitate and not be polite and shake their hand. I also don't want to get hives from peanuts I just saw them eat. It is a fine line between being polite and keeping myself safe!
If I have just seen someone eating something that I am allergic to, I usually just tell them. I am not going to shake their hand whether it was nice to meet them or not! I don't want to take that kind of risk. Most of the time, there really is no way of knowing. I carry wipes with me at all times, but that is really all that I can do!