Lymph nodes are small soft bundles of lymphatic tissue shaped in nodules and are not usually visible or easily felt. Approximately 600 of these minute structures are located in clusters at various parts of the body such as under the armpits, behind the ears, in the groin, behind the knees and in other areas. Varying in size from microscopic to about one inch in diameter, these structures are responsible for filtering lymph fluid, producing plasma cells, and evacuating foreign material such as bacteria. Along with bone marrow, lymph nodes produce the white blood cells that form lymphocytes. One of the most studied varieties of antibody producing white blood cells are the T lymphocytes also referred to as T-cells.