Could alien blood be toxic to us?

Could alien blood be toxic to us? It may seem like a funny question, but it leads to some interesting scientific places!

Not surprisingly, the chances that exo-toxicology (the toxicology of extraterrestrial life forms) becomes a critical issue in our lifetimes is small. But with rapid developments in fields like synthetic biology, it may not be long before we have to ask serious questions about the risks and the potential harm associated with home grown "alien" life forms.
As usual, don't forget to share your thoughts and ideas in the comments below.