What to do if you have a child with a swollen gland under the arm?

Be seen: It sounds like a reactive lymph node or a small sebaceous gland infection. He/she should be seen to determine what is going on with them.

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.