I found this info online from Dr Dezube (Harvard Medical School)...
"Health care professionals, including myself, use the word "reactive" frequently. A "reactive" lymph node is one which is enlarged because it is trying to fight off an infection. The truth is that the only way for one to really know if a lymph node is reactive (enlarging as it's fighting off an infection) or malignant is to biopsy the lymph node. That said, radiologists as well as clinicians see lymph nodes day in and day out. After a while, we get a sense of which lymph nodes are worrisome and which are not. On CT scans, we seldom get excited about lymph nodes less than 1 cm."