Immediately is an adverb describing an action occurring without delay. Grammar.com's section on the Parts of Speech discusses the discussion around -ly adverbs.
"5 hours ago — immediately - adverb. Grammar.com's section on the Parts of Speech discusses the demise of -ly adverbs. Click here for that discussion."
A concise note clarifies the spelling and points readers to a broader discussion about -ly adverbs without adding further examples or rules.