Why is "i" the unit for blood type?

Answer 1

Are you asking in terms of genetics? "i" is often used as the symbol for the allele which determines blood type O. The other two alleles are I^A and I^B which are both dominant to i.

I'm not sure the historical origin of these symbols...

Answer 2

The symbol "i" is not a type of blood. In medicine, that symbol stands for the number "1" (one). A single vertical line with a dot above it means to administer "one" of whatever is ordered. So, 2 vertical lines with a dot above each would mean to administer "two" of whatever is being ordered. Therefore, a doctor's order reading, "Whole blood, AB- i IV now". would mean to deliver one pint (unit) of whole blood, type AB negative, through an IV line now. I hope this helps.