Ants are in no way harmful to your vegetable plants. With at least one caveat. They love to raise and promote aphids. So cute, really. Lettuce is easy 'food' for aphids but rarely will you see aphids on lettuce unless there is nothing else available and/or they've been PUT there by ants!
Look for tear drop shaped crawling insects. They can be bright white to the darkest black.
For your lettuce I would simply harvest a salad and wash the leaves well. The rest might need some spraying of a mild pesticide such as soap and water or a properly mixed NEEM preparation. Spray at night so that bees are not sprayed, okay? Even if the package says, safe for bees, do not believe it.
Otherwise, the ants are 'considering' your lettuce for a herd of aphids. Simply harvest leaves, shake them out before cleaning. When you see aphids, THEN you can use the soap water or neem treatment. Not before.
I live in a world of ants. All kinds of ants. Some sting. They are in my big greenhouse in the soil aerating and pooping and they are great for soil. I think they might have been trying out a different environment and brought aphids into my little grow house under artificial lights. No other reason for them to be there, the starts aren't at all on aphids fav food lists. Only 2 out of 30 plants had aphids. To me that says the ants brought them in and put them on 2 plants to propagate. Full of aphids. Ants are smart...
Update: If there are no aphids at all then I believe Fuzzy Chef's answer is the correct one. Same thing, just shake them and the wash them. Insects not only drink water they poop (fracas) so washing your vegeys is a good thing. Never thought about insects drinking water on purpose. Love this epiphany.