What do hard, swollen, extremely painful/tender lymph nodes in the neck mean?

Diff possibilities: Tender is usually a good sign & if you have a few relatively small tender lymph nodes & you struggle with allergies or have a cold, those could cause it. However if you have a very lg or hard, immobile node(s) it's best to get that checked out. You might also want to check for swollen lymph nodes elsewhere in the body & if you find some, that would be another reason to get checked out.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad 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.