Lymph nodes play an important part in the body's defense against infection. Swelling might occur even if the infection is trivial or not apparent. Swelling of lymph nodes generally results from localized or systemic infection, abscess formation, or malignancy. Common areas where the lymph nodes can be felt include the groin area, armpit, the neck, under the jaw and chin, behind the ears, and below the occiput (prominence on the back of the head). As a rule, when swelling appears suddenly and is painful, it is usually caused by injury or an infection. Enlargement that comes on gradually and painlessly may result from malignancy or tumor.