The lymphatic system is a vital part of the immune system, along with the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine. The lymphatic system is a network of lymph nodes connected by lymphatic vessels. This system transports lymph throughout the body. Lymph is formed from fluid that seeps through the thin walls of capillaries into the body's tissues. This fluid contains oxygen, proteins, and other nutrients that nourish the tissues. Some of this fluid reenters the capillaries and some of it enters the lymphatic vessels (becoming lymph). Small lymphatic vessels connect to larger ones and eventually form the thoracic duct. The thoracic duct is the largest lymphatic vessel. It joins with the subclavian vein and thus returns lymph to the bloodstream. Lymph also transports foreign substances (such as bacteria), cancer cells, and dead or damaged cells that may be present in tissues into the lymphatic vessels and to lymph organs for disposal. Lymph contains many white blood cells. All substances transported by the lymph pass through at least one lymph node, where foreign substances can be filtered out and destroyed before fluid is returned to the bloodstream. In the lymph nodes, white blood cells can collect, interact with each other and with antigens, and generate immune responses to foreign substances. Lymph nodes contain a mesh of tissue that is tightly packed with B cells, T cells, dendritic cells, and macrophages. Harmful microorganisms are filtered through the mesh, then identified and attacked by B cells and T cells. Lymph nodes are often clustered in areas where the lymphatic vessels branch off, such as the neck, armpits, and groin.