Lymph is a clear to yellowish watery fluid which is found throughout the body. It circulates through body tissues picking up fats, bacteria, and other unwanted materials, filtering these substances out through the lymphatic system. It is sometimes possible to see your own lymph; cuts sometimes weep clear lymph rather than blood, for example. The circulation of lymph through the body is an important part of immune system health.

This clear fluid contains white blood cells, known as lymphocytes, along with a small concentration of red blood cells and proteins. Because white blood cells are smaller than red blood cells, they can pass easily through membranes which red blood cells cannot penetrate. The lymph circulates freely through the body, bathing cells in needed nutrients and oxygen while it collects harmful materials for disposal. You could think of lymph as the milkman of the body, dropping off fresh supplies and picking up discarded bottles for processing elsewhere.

As lymph circulates, it is pulled into the lymphatic system, an extensive network of vessels and capillaries which is linked to lymph nodes, small nodules which act as filters to trap unwanted substances in the lymph. Lymph nodes also produce more white blood cells, refreshing the lymph before it is pumped out of the lymphatic system and back into the body. Lymph may not be as showy as blood, but it is related to an equally complex and ornate system of vessels.

Lymph also explains why things like intramuscular shots at the doctor's office work. When the doctor injects a substance into your muscle tissue, the substance is picked up by the lymph and then slowly filtered into the bloodstream. You may also have noticed that when you wear tight clothing or your circulation is otherwise impeded, fluids build up, causing edema, which can be both painful and dangerous. Edema happens when lymph cannot circulate to pull these fluids out.

The lymphatic system can sometimes be used as a diagnostic tool to help doctors understand disease. Lymph nodes can be biopsied, for example, to collect evidence about bacterial agents and toxins in a patient's body. Some types of bodywork are also designed to promote the healthy circulation of lymph to encourage drainage and healthy tissues. Lymphatic massage and other types of bodywork require special training, as a massage therapist can inadvertently cause lymphedema, a collection of fluid on a limb which can become dangerous and extremely painful.