The original source of lymph is blood plasma.

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Lymph is essentially a clear, colorless fluid that is derived from the interstitial fluid, which surrounds the body's cells and tissues. This interstitial fluid is derived from blood plasma, as a result of the fluid that is filtered out of the blood capillaries and into the surrounding tissues. As the interstitial fluid accumulates in the surrounding tissues, it is eventually collected by small lymph vessels, which then carry it back into the bloodstream, completing the cycle of fluid circulation in the body.

The Original Source of Lymph Is Blood Plasma