What makes blood special?

It has become a pet peeve of mine whenever someone tries to come up with a science-fictiony explanation for why vampires exist in a story. It almost always ends up sounding really lame to anyone who has the slightest idea what blood is made of and how metabolism works. And yet, I feel the need to provide a science-fictiony explanation for why my protagonist becomes a vampire after being infected with nanodevices.

The difficulty is that blood is a fascinating, complex tissue, but I can't think of a single thing about it that's unique. It's high in iron, but so is muscle and so are some vegetables. It's rich in protein, but so is every living thing! It's full of immune cells, but so are lymph nodes and bone marrow. It contains a lot of heme groups, but so do the cytochromes, which are found in mitochondria, which are found in EVERY SINGLE CELL IN OUR BODIES!

To make things even more complicated, I want it to be necessary to ingest the blood. I don't want my protagonist to gain the same benefits from a blood transfusion. That eliminates hormones and other signaling molecules that float around in the blood. This train of thought makes me think that something produced in the digestion of blood allows her to function (more than function, in fact - I see her being enhanced by the experience). 

I shall have to ponder this further.