Hi Char,
According to this site:
www.reocities.com/HotSprings/Oasis/6455/western-blot.txt Band 18 is a flagellin (tail) fragment of a spirochete (type of bacteria that include Lyme, syphilis and a few others). Although it's one of the first bands to turn positive in a Lyme infection, unfortunately it can cross react to other things like syphilis, Pintas and Yaws, Relapsing tick fever and others.
Did you have any other bands that were positive or marked as "IND"?