Lymph Node Diseases. The lymph nodes are part of the lymphatic system. They have several important functions regarding the immune system, and are good indicators of disease. The mammalian body has many internal lymph nodes. These can only be visualized by exploratory surgery, a CT scan, an MRI, a radiograph, or an ultrasound. The most common cause of why lymph nodes swell are infections. Any type of infection can cause this reaction in the immune system, but the most common are: mononucleosis, ear infection, gingivitis, tonsillitis, dental abscesses, tuberculosis or skin infections. How does swollen lymph node feel like?.