Firstly get thorough general physical examination to see the presence of any other lymph
nodes inthe body.
You need an X ray
of chest, a smear test from the throat, complete blood count
with differential leukocyte count with peripheral smear examination to find if there is any abnormal cell and also if it is of infectious nature.
If no conclusion through these tests a fine needle aspiration from the node of the neck to rule out any neoplastic process.
It is always better to treat the cause than the symptom.