Choose an activity under What happens? and follow the defence step by step. White dots are white blood cells, green dots travel in the lymph, yellow dots are antibodies and orange dots are chemical signals. The close-up in the corner shows what the cells are doing.
The immune system is not one organ: it is white blood cells and the organs where they are made, trained and gathered, linked by the blood and by the lymph vessels. Click an organ in the model or a colour in the key.
Infect the body, give a vaccine or a course of antibiotics, and watch the germs, the antibodies and the memory cells. Above the dashed red line you feel ill. Start with a preset.
Simplified teaching model with relative levels, not medical advice. Real illnesses differ in timing.
The 3D model above shows the organs inside the body; this flat diagram is the version you draw and label in an exam. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the diagram, sort the defences, match the cells, multiple choice and explain questions, with an answer key.
Most germs never get in. Skin is a tough barrier, mucus in the nose and airways traps germs and cilia sweep them away, stomach acid kills germs we swallow, and tears and saliva contain an enzyme (lysozyme) that damages bacteria. This is the first line of defence.
When germs get past the barriers, for example through a cut, the innate (second-line) defences start within minutes. Inflammation makes the area red, warm and swollen, bringing more blood. Phagocytes, white blood cells such as neutrophils and macrophages, engulf and digest the germs (phagocytosis). A fever can help too. These defences work the same way against every germ.
Every germ carries molecules called antigens on its surface. In the lymph nodes, the few B cells and T cells whose receptors fit a germ’s antigen are switched on and multiply. B cells become plasma cells that make antibodies, Y-shaped proteins that fit only that antigen; killer T cells destroy infected body cells. Afterwards, memory cells stay for years, so a second infection is stopped much faster. A vaccine makes memory cells without the illness.
Run A cut gets infected and then Lymph node: antibodies to see the second and third lines one after the other; watch the close-up and the armpit nodes swell. Then in the lab press Catch it again: the antibody peak after the second infection is several times higher and comes much sooner. Finally compare Antibiotics (bacteria) with Antibiotics on a virus.
| Line | What it is | Examples | Speed |
|---|---|---|---|
| 1st: barriers | Stop germs getting in | Skin; mucus and cilia in the airways; stomach acid; lysozyme in tears and saliva; coughing and sneezing | Always on |
| 2nd: innate response | Attacks any germ that gets in, the same way every time | Inflammation; phagocytes (neutrophils, macrophages); natural killer cells; fever; interferons | Minutes to hours |
| 3rd: adaptive response | Targets one particular germ and remembers it | B cells and antibodies; helper T cells; killer T cells; memory cells | About 1–2 weeks the first time; days the next time |
| Organ | Where | Job |
|---|---|---|
| Red bone marrow | Inside bones (in adults mostly hip bones, breastbone, ribs, spine and the upper ends of the thigh and upper-arm bones) | Makes all blood cells, including every white blood cell; B cells mature here |
| Thymus | Upper chest, behind the breastbone | T cells mature here; most active in childhood, largest around puberty, then shrinks |
| Lymph nodes | Hundreds, in clusters in the neck, armpits, chest, abdomen and groin | Filter lymph; B and T cells meet antigens and are switched on |
| Spleen | Left side of the upper abdomen | Filters the blood; removes old red cells; makes antibodies |
| Tonsils & adenoids | Back of the mouth and nose | Sample germs that are breathed in or swallowed |
| Lymph vessels & thoracic duct | All through the body | Carry tissue fluid (lymph) through the nodes and back into the blood |
| Appendix & Peyer’s patches | Intestines | Lymphoid tissue that guards the gut |
| Cell | Line | What it does |
|---|---|---|
| Neutrophil | Innate | The most common white blood cell; a phagocyte that arrives first at an infection and engulfs bacteria |
| Macrophage | Innate | A large phagocyte (grows from a monocyte) that engulfs germs and dead cells and shows antigens to T cells |
| Dendritic cell | Link | Picks up antigens in the tissues and carries them to the lymph nodes to switch on T cells |
| Natural killer cell | Innate | Kills virus-infected cells and some cancer cells without needing to recognise one particular antigen |
| B cell / plasma cell | Adaptive | B cells that meet their antigen become plasma cells that make antibodies, or memory B cells |
| Helper T cell | Adaptive | Recognises an antigen and sends signals that switch on B cells and killer T cells |
| Killer (cytotoxic) T cell | Adaptive | Destroys body cells infected by a virus, and some cancer cells |
| Memory B and T cells | Adaptive | Stay for years and respond fast if the same antigen comes back |
| Primary response (first time) | Secondary response (next time) | |
|---|---|---|
| Delay before antibodies | About 1 week | A few days |
| Amount of antibody | Lower | Much higher |
| How long it lasts | Shorter | Longer |
| Do you feel ill? | Often yes | Usually no: the germ is destroyed early (you are immune) |
A vaccine trains the adaptive immune system safely. It contains killed or weakened germs, pieces of a germ such as one of its proteins, or instructions (mRNA or a harmless carrier virus) that let your own cells make one germ protein for a short time. The body makes antibodies and memory cells without having the disease; some vaccines need a booster dose to keep the memory strong. When most people in a community are immune, a germ cannot spread easily, which also protects people who cannot be vaccinated: this is herd immunity.
Antibiotics kill bacteria or stop them multiplying, but they do not work against viruses such as colds and flu. Using them when they are not needed helps bacteria become resistant.
The adaptive response has two arms. In the humoral response, B cells make antibodies that act on germs in the blood and lymph. In the cell-mediated response, killer T cells destroy infected cells. T cells can only recognise antigen fragments displayed on the cell surface by MHC proteins: helper T cells (CD4) see antigens on antigen-presenting cells such as dendritic cells, macrophages and B cells, and killer T cells (CD8) see antigens on infected cells. Because each B and T cell has its own receptor, made by randomly shuffling gene segments, the body holds a huge range of receptors before any infection; an antigen selects the few cells that fit, which then multiply (clonal selection). Immunity you make yourself, after an infection or a vaccine, is active immunity; antibodies received from someone else, across the placenta or in breast milk, give short-lived passive immunity. See how an antibody’s shape fits its antigen in the 3D antibody simulation.
The main organs are the red bone marrow, where all white blood cells are made; the thymus, where T cells mature; the lymph nodes, which filter lymph and where B and T cells are switched on; the spleen, which filters the blood; and the tonsils, adenoids, appendix and Peyer’s patches, which guard the throat and gut. They are linked by the blood and by the lymph vessels.
Key takeaway: bone marrow, thymus, lymph nodes, spleen, tonsils and gut lymphoid tissue.The first line is the barriers that keep germs out: skin, mucus and cilia, stomach acid and the enzyme lysozyme in tears and saliva. The second line is the innate response that attacks any germ that gets in: inflammation, phagocytes and fever. The third line is the adaptive response: B cells and antibodies and T cells that target one particular germ and leave memory cells.
Key takeaway: barriers, then a general attack, then a targeted attack with memory.The lymphatic system is a network of thin lymph vessels and lymph nodes. It drains the extra tissue fluid that leaks out of blood capillaries, filters it through the lymph nodes, where white blood cells check it for germs, and returns it to the blood through a vein near the collarbone. It also absorbs fats from the small intestine.
Key takeaway: it drains tissue fluid, filters it for germs and returns it to the blood.Phagocytes such as neutrophils and macrophages engulf and digest germs (phagocytosis). B cells, after being switched on, become plasma cells that make antibodies, which stick to germs, clump them and mark them for destruction. Killer T cells destroy body cells that are infected by viruses, and helper T cells coordinate the response.
Key takeaway: they engulf germs, make antibodies or kill infected cells.An antigen is a molecule, usually on the surface of a germ, that the immune system recognises as foreign. An antibody is a Y-shaped protein made by B cells (plasma cells) whose shape fits one particular antigen, like a key in a lock. Antibodies bind to the antigen and help to destroy the germ.
Key takeaway: the antigen is on the germ; the antibody is made by you to fit it.During an infection, dendritic cells bring antigens from the infected area to the nearest lymph nodes. There the matching B and T cells are switched on and multiply very fast, and more cells and fluid arrive, so the nodes get bigger and can feel tender. These are the “swollen glands” in the neck during a sore throat.
Key takeaway: B and T cells (lymphocytes) multiply in the node to fight the infection.A vaccine contains a harmless form of a germ or of one of its antigens, or instructions for your cells to make one antigen for a short time. The immune system responds as it would to the real germ, making antibodies and memory cells, but without the disease. If the real germ arrives later, the memory cells make a fast and large secondary response that usually stops it before you feel ill.
Key takeaway: vaccines make memory cells without the illness.Colds and flu are caused by viruses. Antibiotics work by attacking structures and processes found in bacteria, such as their cell walls, which viruses do not have, so antibiotics have no effect on viruses. Using antibiotics when they are not needed helps bacteria become resistant. Viral infections are fought by the immune system, and some can be treated with antiviral medicines.
Key takeaway: antibiotics kill bacteria, not viruses.After the first infection, memory B and T cells for the chickenpox virus remain for many years. If the virus is met again, they produce a secondary response that is much faster and larger, so the virus is destroyed before it can cause the disease. This is immunity. Colds and flu are different because there are many different cold viruses and flu viruses keep changing their antigens.
Key takeaway: memory cells stop the same germ the second time.Innate immunity is present from birth, responds within minutes to hours, attacks any germ in the same way and does not improve with repeated exposure. Adaptive immunity develops during life, takes days to respond the first time, is specific to one antigen and has memory, so it responds faster and more strongly next time. The two work together: innate cells such as dendritic cells switch on the adaptive response.
Key takeaway: innate is fast and general; adaptive is slower at first, specific and remembers.