What causes regional neck lymphadenitis in the absence of cancer and bacteria?

Many possibilities. Any number of viruses do this. If prolonged more than a couple of weeks, maybe mononucleosis, although you're quite old for mono (most cases before age 20). Mostly not serious, but syphilis, HIV, tuberculosis, land other serious infections are possible if at risk. If only a few days, sit tight; probably will clear up soon. If continuing more than 2 weeks, see your doctor. Good luck!