My 2.5 year old son has swollen lymph nodes in neck, clavicle, underarm and groin. only swollen on the left side, fever started 2 days ago.

Lymphadenopathy: At this age, we see fever with certain group of lymph node enlargement, like neck, under chin, behind ears and some other areas. Cause is mostly of viral. If multiple groups are involved with fever but no rash, i will advise to have a thorough exam by his pediatrician, check for liver /spleen enlargement, have simple blood test like a cbc. With diff. And any more if needed. Do not get scared.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells/digested food. Lymph nodes "taste" the lymph everywhere for bad stuff like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.