The lymphatic system is part of our immune system, and runs alongside the blood vessels and arteries that circulate our bodies. It funnels nearly 3 liters of fluid through its vessels in each day. The fluid is called lymph and is made up of the watery substance that is left over after blood capillaries absorb the blood that they need (for tissue and cells). Lymph does not contain any red blood cells and lymphatic vessels send the left-over fluid back toward the heart so it can re-enter the blood stream.
The lymph goes through many checkpoints on its way back, called lymph nodes. We have lymph nodes in our neck, under our arms and in various places throughout the body. We also have larger lymphoid organs like the spleen, thymus and tonsils. Lymph nodes and other lymphoid tissues can become swollen and inflamed when they are fighting an infection or antigen. The tonsils surround the trachea in the back of the mouth and that's why you may experience a sore throat when you have a cold.
Lymph nodes contain many immune system cells called lymphocytes. Our appendix, at the end of the colon, also houses lymphocytes to destroy remaining bacteria in our feces. Lymphocyte cells called T-cells directly attack infections, B-cells secrete antibodies (into the blood) and macrophages eat up foreign substances, all protecting our bodies from the harmful waste that enters it.