Lymphatic system · immunity
The lymphatic system — a silent army
Alongside our blood vessels runs a second, lesser-known network. It collects fluid from the tissues, carries fats from the gut and delivers microbes straight to hundreds of lymph nodes — outposts where an army of white blood cells lies in wait. See how swelling forms, how eater cells defend a cut, how lymphocytes learn a new enemy, and why vaccines protect whole communities.
This page is educational and is not medical advice. The models are simplified; the descriptions follow textbook immunology and WHO guidance. For health matters, see a doctor.
Map of the lymphatic system
Click an organ in the drawing or its name. The green streams are lymph — it always flows one way: from the tissues toward the heart.
Parts of the system
Why do lymph nodes swell?
During a throat infection, lymph carries microbes and their fragments to the nodes in the neck. Lymphocytes that recognize the intruder start dividing rapidly there, and more blood and immune cells flow into the node. The node grows and may feel tender — a sign that work is under way. After the infection it usually shrinks back to its old size.
A node that stays enlarged for several weeks without an obvious reason is worth showing to a doctor.
Fat travels in lymph
Most dietary fats do not go straight into the blood: in the intestinal villi they are absorbed by lymphatic vessels, and lymph from the gut turns milky white (this is chyle) and flows through the cisterna chyli and the thoracic duct into a vein.
Where does lymph come from?
In a capillary, blood pressure pushes water out into the tissues, while plasma proteins (oncotic pressure) pull it back. Not everything returns — the surplus, together with proteins that escaped from the blood, is taken away by the lymphatic system. Block it and watch the tissue.
Starling forces
Fluid balance (whole body)
Lymph has no heart
Blood is pumped by the heart, but lymph is moved by: contracting skeletal muscles that squeeze the vessels, valves that keep it from flowing back, breathing (each breath in draws lymph deeper into the chest) and rhythmic contractions of the walls of the larger lymphatic vessels.
What is the leg's owner doing? L
That's why after a whole day on your feet or a long flight your shoes can feel tight in the evening. Movement helps — even circling your feet while sitting — and so does raising your legs, because then gravity doesn't fight the return of fluid.
First line: barriers
Before any cells join the fight, most microbes are stopped by walls and traps we are born with.
Infection in a cut
Bacteria have slipped in through the broken skin. Click or hold your finger where they multiply — you'll send a chemical signal (chemokines) that summons the eater cells. Neutrophils arrive first; larger macrophages clean up.
Progress
Signs of inflammation
- Redness — histamine from mast cells widens the blood vessels, so more blood flows to the injury.
- Warmth — warm blood from the body's core flows in faster.
- Swelling — vessel walls become leaky, and plasma with proteins seeps into the tissue (so the lymph has more to drain).
- Pain — inflammatory substances (e.g. bradykinin, prostaglandins) and the pressure of swelling irritate nerve endings.
Fever is also a defense: the hypothalamus turns the body's “thermostat” up, which slows the growth of many microbes and speeds up immune cells.
Pus is mostly dead neutrophils and debris of bacteria and tissue. Antibiotics act on bacteria, not viruses, and overusing them encourages drug resistance — see natural selection.
A story from a lymph node
Adaptive (specific) immunity learns a particular enemy. Seven scenes: from a scout bringing news to memory cells. ← → switch scenes.
Scene 1 of 7
A scout brings news
Clonal selection — find the matching B cell
Every B cell carries receptors of one shape of its own. An antigen fits only some of them — like a key in a lock. Click the cell whose receptor is an exact mold of the antigen (with the keyboard: arrow keys and Enter).
Antigen
Look for a receptor whose notches are filled exactly by the antigen's bumps.
Why so many shapes? As each lymphocyte matures, it randomly “shuffles” pieces of its receptor genes (V(D)J recombination). That way the body has receptors ready for antigens it has never seen before. When one fits, that single cell divides over and over, and after a few days it forms thousands of identical copies — a clone.
First and second response
First encounter with an antigen: a slow start and a moderate level of antibodies. Second: memory cells react within a few days, and there are many times more antibodies. This is how vaccines work.
Contacts with the antigen
Click “1st contact”, then “2nd contact”. The units are arbitrary — the shape of the curves is typical, but the exact values depend on the antigen and the person.
Allergy
The immune system overreacts to a harmless antigen (an allergen) such as pollen, pet dander or peanuts: it makes IgE antibodies that arm mast cells, and on the next contact these release histamine — hence the runny nose, itching and rash.
Autoimmunity
The immune system mistakenly attacks the body's own tissues because the “school of tolerance” in the thymus and bone marrow has failed. This is how type 1 diabetes (destruction of the insulin-producing cells of the pancreas) and celiac disease, among others, arise.
How does a vaccine work?
A vaccine shows the immune system a safe “wanted poster” — the antigen — before it meets the real microbe. Memory cells form, so when infection comes, the response is fast and strong, like the second response on the chart in the Adaptive immunity mode.
- Live attenuated
A weakened (attenuated) microbe that multiplies very poorly and does not cause disease in a healthy person.
e.g. measles, mumps, rubella (MMR) - Inactivated
Killed microbes — they cannot multiply, but their proteins still teach the immune system.
e.g. polio (IPV), rabies - Subunit and toxoid
Just a selected fragment: a protein from the microbe's surface or an inactivated toxin (a toxoid).
e.g. hepatitis B, tetanus - mRNA
Instructions inside a tiny droplet of fat: cells use them to build one viral protein, and the mRNA quickly breaks down. It does not enter the cell nucleus and does not change DNA.
e.g. COVID-19 - Viral vector
A harmless carrier virus (e.g. an adenovirus unable to multiply) delivers the instructions for building one protein.
e.g. some COVID-19 vaccines
Herd immunity — a simulation
Each dot is a person. Set how contagious the disease is (R₀) and the share of people vaccinated, then let in a few infected people from outside. Simplified model: the vaccine is 100% effective, the illness lasts 5 days, and contacts are mostly neighbors.
Disease and vaccination
Result
Let the sick in and see how far the epidemic spreads.
In reality vaccines do not protect everyone 100% — for example, two doses of the measles vaccine protect about 97% of those vaccinated — which is why WHO recommends that at least 95% of children receive both doses. Herd immunity also protects those who cannot be vaccinated: infants and people with weakened immunity. Vaccines, like any medicine, go through clinical trials and are continuously monitored; the most common reactions are a sore arm or a brief fever.
From the history of vaccination
WHO estimates that vaccination prevents 3.5–5 million deaths every year.
- 1796
Edward Jenner introduces material from cowpox blisters into 8-year-old James Phipps and shows that it protects against smallpox. The word “vaccine” comes from the Latin vacca — cow.
- 1885
Louis Pasteur gives the rabies vaccine to 9-year-old Joseph Meister, who had been bitten by a rabid dog. The boy survived.
- 1920s–1940s
In Lwów (now Lviv), Rudolf Weigl creates a vaccine against epidemic typhus by growing the germs in the guts of lice, which people fed with their own blood — the “lice feeders.” During the war, working at his institute shielded many scientists from persecution, and the vaccine was secretly smuggled into places including the ghettos. Weigl was honored as Righteous Among the Nations.
- 1950
Hilary Koprowski, born in Warsaw, tests a live oral polio vaccine on a human for the first time. In the late 1950s it was used, among other places, in Poland for mass vaccination of children.
- 1963
A smallpox outbreak in Wrocław (99 cases, 7 deaths) is brought under control by isolating the sick and vaccinating on a mass scale.
- 1980
WHO declares smallpox eradicated — the first human disease defeated by vaccination. The last natural case was recorded in Somalia in 1977.
How does it work?
The lymphatic system is a network of thin vessels, lymph nodes and organs (spleen, thymus, tonsils, bone marrow, Peyer's patches). It has three jobs: it returns excess fluid from the tissues to the blood, carries fats absorbed in the gut and — most importantly — it is the home and highway of the immune system.
Starling forces
Whether water leaves a capillary or returns to it is decided by a difference in pressures: hydrostatic (pushes water out) and oncotic (proteins pull water in). At the start of the capillary filtration wins; at the end, reabsorption.
Two lines of defense
Innate immunity acts right away and in the same way against many microbes: barriers, inflammation, eater cells (neutrophils, macrophages), NK cells, fever. Adaptive immunity needs a few days but is precise: B cells make antibodies that fit one antigen, helper T cells (CD4) coordinate the response, cytotoxic T cells (CD8) kill infected cells, and memory cells remain for years.
Herd immunity
If each sick person infects on average R0 people in a population with no immunity, then with a fraction p immune they infect only Re people. The epidemic dies out when Re < 1.
Simplifications: the fluid balance is a model of an “average” capillary scaled up to the whole body; tissue compliance, the rise in lymph flow and the washout of proteins from the tissue are approximated by simple relationships. Newer research (the glycocalyx layer) suggests that in many tissues reabsorption is smaller than the classic model says — so the role of lymph is even greater. The lymph node story compresses days into seconds and leaves out many cells and molecules. The epidemic simulation assumes lifelong immunity and a 100% effective vaccine.
Keys
- Q W E R — modes (number keys work within modes)
- Space — pause / resume animations
- 1 2 3 — capillary situations (Q)
- L — walking / standing still / legs up (Q)
- N — new cut (W), new antigen (E), new population (R)
- ← → — story scenes (E)
- 1–4 — diseases, S — let the sick in (R)
- On the boards: arrow keys + Enter
See also
Did you know…?
- Until 2015 it was thought that the brain has no lymphatic vessels. Then two independent research teams discovered them in the meninges surrounding the brain.
- You have only about 20,000 protein-coding genes, yet your lymphocytes can produce billions of different receptors — thanks to the shuffling of gene pieces (V(D)J recombination).
- The thymus, the “school” for T cells, is largest in childhood. After puberty it gradually shrinks and is replaced by fatty tissue, but the lymphocytes trained there serve for years.
Challenges
- Cause swelling by blocking lymph drainageMode Q: reduce lymph drainage to 10% or less and produce visible swelling (at least +2.5 L of fluid in the tissues).
- Help the macrophages beat the infectionMode W: summon eater cells with signals until not a single bacterium is left in the wound.
- Find the B cell that matches the antigenMode E: pick the cell whose receptor is an exact mold of the antigen.
- Stop an epidemic with herd immunityMode R: with R₀ ≥ 3, vaccinate enough people that fewer than 2% of the population fall ill.
- Compare the first and second immune responseMode E: trigger both contacts with the antigen and see the whole second response on the chart.
Laboratorium · The lymphatic system and immunity. Educational content, not medical advice.