Reactive hyperplasia of underarm lymph nodes may be associated with inflammation, tumor invasion and other diseases in the body. Lymph nodes are one of the components of the body’s immune system, when the body is subjected to a variety of injuries, stimulation, it will cause the lymphocytes and tissue cells in the lymph nodes reactive hyperplasia, resulting in lymph node enlargement. Reactive hyperplasia of axillary lymph nodes mainly reflects inflammation in the surrounding drainage area, tumor invasion or connective tissue disease. Axillary lymphadenitis is a common manifestation of reactive hyperplasia. For example, axillary folliculitis, mastitis, pneumonia, etc. may cause axillary lymph node enlargement. And breast cancer, lung cancer, thyroid cancer, nasopharyngeal cancer, etc. can also be metastasized through the lymph, resulting in enlarged axillary lymph nodes. Lymphoma, lymphoblastic leukemia, etc. can also invade the axillary lymph nodes, resulting in enlargement of these lymph nodes. Systemic lupus erythematosus, tuberculosis, etc. can also invade the axillary lymph nodes leading to enlargement. There are other possible causes of axillary lymph node hyperplasia, which should be promptly examined in the hospital and treated according to the doctor’s instructions after a clear diagnosis.