Lymph nodes are small, round organs of the lymphatic system that support proper functioning of the immune system. They help the body to fight off infection by filtering foreign particles out of lymph, a clear or whitish fluid that is made up of white blood cells. Lymph also contains a type of white blood cell called lymphocytes, which help attack bacteria in the blood. Mediastinal lymph nodes are glands that are located in the part of the chest that lies between the sternum and the spinal column. This region is referred to as the mediastinum, and contains the heart, thymus gland, windpipe, and large blood vessels. Mediastinal lymph nodes are responsible for helping bone marrow and the thymus produce mature lymphocytes. Lymph nodes vary in size from the size of a pinhead to the size of a lima bean. They are enclosed in a fibrous capsule. Lymph nodes are connected to each other by various lymphatic vessels and are efferent vessels (meaning away from the center or away from the central nervous system).