What is lymph?

Tissue fluid. Arteries and veins carry blood cells and a serum, some of which leaks out from capillaries and bathes your tissues. It gradually accumulates and is collected in lymph channels which empty back into the heart. It traverses nodes (lymph nodes) which can be the first line of defense against infection and cancer. When blocked, like after lymph node dissection, it can accumulate causing lymphedema.
Clear fluid. It is a fluid (usually clear) that circulates between the tissues of your body. It is fluid not in the arteries and veins. It contains some proteins and white blood cells.

Related Questions

What causes lymph nodes to swell?

Sink traps. Lymph nodes are sink traps for our bodies. If you have infection or inflammation somewhere, your friendly hardworking lymph nodes will catch bits, work on it, & swell. They may feel tender. They may not hurt at all. Example: common cold & swollen neck lymph nodes. See your doc if you're concerned, if there's not an obvious neighborhood reason why your nodes are swollen, if it's more than 3-4 weeks.
Swollen nodes. Nodes can swell because of infection or inflammation (some people refer to lymph nodes and tonsils as "glands") or because of local obstruction of their drainage (as in the salivary glands). So the best answer would depend on to which glands or nodes you are referring.

Could you tell me what are lymph nodes and are they threatening?

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.

What causes lymph nodes on a child's neck to swell up occasionally?

Normal. Lymph nodes swell up when they are doing their job correctly. They help to fight inflammation and will enlarge when doing so. They are aiding your baby's immunity.
Common finding. Lymph nodes are located throughout the body ; generally noticeable in exposed areas like the neck. They are usually pea sized at rest and enlarged when stimulated by bug bites, dermatitis, drainage or infection. They usually return to resting size in 3-4 wks. They rarely can become damaged by infection ; grow to golf ball size ; need drainage or removal.
Infections, allergies, dermatitis are the more common causes. The nodes will usually resolve in a couple of weeks.

What enlarges lymph nodes on side of neck?

Many things. Lymph node enlargement (lymphadenopathy) may occur due to a large variety of factors like infections (bacterial, viral or fungal), immune disorders, drug reactions or malignancy (lymphomas, carcinomas). Correlation with your clinical history and a biopsy usually helps in further characterization.
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.