The Lymphatic and Immune Systems

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.

The Structures

Lymph nodes. Small bean shaped filters positioned along the vessels, clustered in the neck, armpits, chest, abdomen and groin. Lymph passes through them and immune cells inside inspect it for organisms and debris. Nodes enlarge and become tender when they are working, which is why swollen glands accompany infection.

Spleen. Left upper quadrant, behind the stomach. It filters blood rather than lymph, removing aged red cells, holding a reserve of blood, and mounting immune responses to organisms in the bloodstream. It is fragile and highly vascular, which makes it a frequent source of serious internal bleeding after blunt trauma to the left side or lower left ribs.

A patient without a spleen, whether from injury or surgery, is more susceptible to certain bacterial infections for life.

Thymus. In the upper chest behind the sternum. T lymphocytes mature here. It is large in children and shrinks steadily through adulthood, since most of that training happens early.

Tonsils and adenoids. Lymphatic tissue positioned at the entrance to the throat, sampling what comes in through the mouth and nose.

Bone marrow. Where all blood cells originate, including every immune cell.

Innate Immunity

Innate immunity is what you are born with. It responds identically to every threat, it acts immediately, and it does not learn.

Physical and chemical barriers come first: intact skin, mucous membranes that trap organisms, cilia that sweep the airway, stomach acid, tears and saliva. Most of what tries to get in never does.

Inflammation is the innate response to tissue damage or invasion. Vessels in the area dilate and become leaky, which brings more blood and more immune cells to the site. That produces the four classic signs: redness, heat, swelling and pain. The response is useful, and it is also the reason a local infection looks the way it does.

Phagocytes, mainly neutrophils and macrophages, engulf and destroy organisms and debris. Fever is another innate response, raising body temperature to a range less hospitable to many organisms and speeding immune activity.

Adaptive Immunity

Adaptive immunity is learned. It targets one specific organism, takes days to mount the first time, and remembers afterward.

Lymphocytes do the work, in two types.

B lymphocytes produce antibodies, proteins that lock onto one specific target, called an antigen. An antibody marks an organism for destruction, neutralizes a toxin, or clumps organisms together. This is humoral immunity, working through the blood and body fluids.

T lymphocytes act directly. Helper T cells coordinate the response and activate other cells. Cytotoxic T cells destroy the body's own cells that have been infected by a virus or turned cancerous. This is cell mediated immunity.

Memory is the reason the second exposure is different. After an infection, memory B and T cells persist, and a repeat encounter produces a response that is faster and stronger, often before you notice symptoms. Vaccination uses that mechanism deliberately, presenting a harmless version of an antigen so memory forms without the illness.

Active immunity is immunity you produced yourself, from infection or vaccination, and it lasts. Passive immunity is borrowed antibodies, as an infant receives across the placenta and through breast milk, and it fades within months.

When the Response Is the Problem

Allergy is an immune response to something that is not actually a threat. First exposure produces antibodies against the substance and no symptoms. Those antibodies attach to mast cells throughout the body. On a later exposure the allergen binds them, and the mast cells release their contents, most importantly histamine.

Histamine does three things at once, and each one produces part of the picture.

• Dilates blood vessels, dropping blood pressure and causing flushing
• Increases capillary permeability, letting fluid leak into tissue, which produces hives, swelling of the lips, tongue and airway, and further loss of circulating volume
• Constricts bronchiole smooth muscle, producing wheezing and difficulty moving air

A mild allergic reaction is limited to the skin: itching, hives, local swelling.

Anaphylaxis is the same process occurring throughout the body at once, and it is defined by involvement beyond the skin, meaning the airway or the circulation. The patient has a distributive shock and an airway emergency simultaneously, which is why it moves so fast.

That physiology explains the treatment order. Epinephrine reverses all three effects directly: it constricts vessels, it reduces leak, and it dilates bronchioles. An antihistamine blocks histamine at the receptor and does nothing about the vessels or the airway quickly enough to matter. A steroid acts over hours. Epinephrine comes first, and everything else is an adjunct behind it.

Autoimmune disease is the immune system attacking the body's own tissue, as in rheumatoid arthritis, lupus and type 1 diabetes. Many of those patients take medications that suppress the immune response, which leaves them more vulnerable to infection and can blunt the usual signs of one.

Immunocompromised patients, whether from disease, chemotherapy, transplant medication or age, may not produce a fever or a normal inflammatory response even with serious infection. A quiet presentation in that population is not reassurance.

What to Take Away

The lymphatic system returns leaked tissue fluid to the blood and houses the immune cells that inspect it. Innate immunity is immediate, identical every time and does not learn. Adaptive immunity is specific, slower on first exposure and remembers. Anaphylaxis is a body wide histamine release producing dilation, leak and bronchoconstriction at once, which is exactly why epinephrine is first and everything else follows it.