English Grammar for the Utterly Confused - Laurie Rozakis 2003
Interjections
Chapter 1. Parts of Speech
Part 1 Welcome to grammarland
Interjections show strong emotion. Since interjections are not linked grammatically to other words in the sentence, they are set off from the rest of the sentence with a comma or an exclamation mark. For example:
• Oh! What a shock you gave me with that gorilla suit.
• Wow! That’s not a gorilla suit!