Lymph is considered a part of the interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...

, the fluid which lies in the interstices
Interstitial space
An interstitial space is an intermediate space located between regular-use floors, commonly located in hospitals and laboratory-type buildings to allow space for the mechanical systems of the building...

 of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary
Lymph capillary
Lymph capillaries or lymphatic capillaries are tiny thin-walled vessels that are closed at one end and are located in the spaces between cells throughout the body, except in the central nervous system, and in non-vascular tissues. The main purpose of these vessels is to drain excess tissue fluids...

. The lymph then travels to at least one lymph node
Lymph node
A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...

 before emptying ultimately into the right or the left subclavian vein
Subclavian vein
The subclavian veins are two large veins, one on either side of the body. Their diameter is approximately that of the smallest finger.-Path:Each subclavian vein is a continuation of the axillary vein and runs from the outer border of the first rib to the medial border of anterior scalene muscle...

, where it mixes back with blood.

Lymph returns protein and excess interstitial fluid to the circulation. Lymph picks up bacteria and brings them to lymph nodes to be destroyed.