Lymph

Vehicle for tissue maintenance and immune defense

Similar to plasma (the extracellular matrix of blood), lymph is a more or less colorless liquid that flows throughout a branched network of vessels, channels and ducts known as the lymphatic system.

Composition, function and transport

Lymph consists mostly of water originating from the interstitial fluid that surrounds cells and bathes the tissues of the body. Because the composition of interstitial fluid is in constant flux due to ongoing substance-exchange with the blood and surrounding cells, the composition of lymph varies accordingly.

Primary functions of the lymphatic system include:

• Maintaining fluid balance within the body by removing excess interstitial fluid to the bloodstream
• Returning proteins and extracellular substances from tissues to the bloodstream
• Transporting fats from the digestive system to the blood
• Filtering and destroying invasive microorganisms

Although humans have no active mechanism for circulating lymph throughout the body, as the heart actively drives the blood, lymphatic flow is established passively by means of body movement and the contraction of skeletal muscles surrounding lymphatic vessels. Lymph circulation is therefore less consistent than blood circulation, although valves present in the larger vessels do keep the fluid flowing in one direction.

The immune connection

Lymph contains high numbers of lymphocytes and macrophages — the immune system’s primary cellular defenses against invasion by bacteria and other microorganisms. Bacteria residing in body tissues are swept up by the lymph and into lymphatic system “hubs” called lymph nodes, where these specialized white blood cells either produce antibodies against bacterial cells or directly attack them by engulfing and degrading them.

Lymphocytes – White blood cells (leukocytes) that are the most numerous of all cells found in lymph and concentrated in lymph nodes; include natural killer (NK) cells, T-cells and antibody-producing B-cells

Macrophages – Leukocytes prevalent in lymphoid tissue and capable of amoeboid movement; actively seek out harmful entities (pathogens, cancer cells, cellular debris, foreign substance) and destroy by engulfing (phagocytosis)

Large amounts of unwelcome material present in the lymph will collect in lymph nodes and trigger dramatic generation of lymphocytes to combat it; as a result, the lymph node can become greatly enlarged and symptomatic of a range of diseases or conditions, including:

• Amyloidosis
• Mononucleosis
• Syphilis
• Tuberculosis
• Anemia
• Acute or chronic lymphocytic leukemia
• Hodgkin’s lymphoma