The Lymphatic and Immune Systems
Where lymph comes from and where it goes, what the spleen, thymus and lymph nodes do, the difference between innate and adaptive immunity, and the physiology behind an allergic reaction becoming anaphylaxis.
8 min read · Updated September 10, 2026
Two Systems That Overlap
The lymphatic system is a network of vessels, nodes and organs that drains fluid from the tissues and returns it to the bloodstream. The immune system is the body's defense against organisms and abnormal cells. They are treated together because the lymphatic network is where much of the immune work physically happens.
Where Lymph Comes From
Blood pressure pushes fluid out of the capillaries into the tissue spaces, which is how cells receive what they need. Most of that fluid is drawn back into the capillaries at their far end. Roughly three liters a day is not, and that leftover fluid is lymph.
Without a route back, that fluid would accumulate and the tissues would swell. Lymphatic capillaries collect it, join into progressively larger lymphatic vessels, and eventually empty into large veins near the collarbones, returning it to the circulation.
Lymphatic vessels have no pump. Movement depends on skeletal muscle squeezing the vessels, on breathing, and on one way valves preventing backflow. That is why a limb that does not move tends to swell.
Edema is what you see when the balance fails: too much fluid pushed out, too little pulled back, or drainage blocked. Swelling in both ankles at the end of the day is gravity plus that balance. Swelling in one arm after lymph node surgery is a drainage problem.
Quick check
A patient's leg has been immobile for several days and is now swollen. Which feature of the lymphatic vessels explains that?
Frequently asked questions
What does the lymphatic system do?
It collects the fluid that leaks out of capillaries into the tissue, filters it through lymph nodes, and returns it to the bloodstream near the collarbones. It also absorbs fat from the small intestine and carries the white cells that mount an immune response.
What is the difference between innate and adaptive immunity?
Innate immunity is what you are born with and it responds the same way to everything: skin, mucous membranes, stomach acid, inflammation and the cells that swallow invaders. Adaptive immunity is learned, it targets one specific organism, and it remembers, which is what makes vaccination and lifelong immunity possible.
What actually happens in anaphylaxis?
A second exposure to an allergen triggers mast cells to dump histamine throughout the body at once. Histamine dilates every vessel and makes capillaries leak, which drops blood pressure, and it constricts the bronchioles and swells the airway. The result is a distributive shock and an airway emergency happening in the same patient at the same time.
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