Ashok,
This happens because Asciidoctor optimizes backslash processing in such a way that it leaves behind some corner cases. The best way to insert a symbol so that it doesn't affect formatting is to use an attribute. Here's how you might write your example:
....
\a\b\c{backslash}__italic__
....
NOTE: You need double underscores around the word italic since it's not separated from the other content (single underscores emphasis a whole word, double underscores emphasize a portion of a word).
If you don't to type out "backslash" in the attribute, you can abbreviate it using an attribute:
....
:bs: \
\a\b\c{bs}__italic__
....
I hope that helps!
-Dan