What are common benign causes of an isolated high lymphocyte count on CBC?

[Emma63]

I recently had a routine complete blood count done, and my results show my lymphocyte count is slightly elevated outside the normal range, though everything else looks fine. My doctor said it's likely nothing to worry about and could be due to a recent mild cold, but I'm a naturally anxious person and can't stop researching potential causes online. For medical professionals or others who understand hematology, what are the most common benign reasons for an isolated high lymphocyte count, and at what point would it typically warrant further investigation like a peripheral smear or follow-up test?