The immune system defends the body against pathogens (bacteria, viruses, fungi and protists) in three layers. First, barriers such as skin, mucus, cilia and stomach acid keep most pathogens out. Second, the innate response attacks anything that gets in, within minutes to hours: phagocytes engulf and digest pathogens, and inflammation brings more of them to the site. Third, the adaptive response targets the specific pathogen: B cells make antibodies that bind to its antigens, helper T cells coordinate the attack, and killer T cells destroy infected body cells. The adaptive response leaves memory cells behind, so a second infection by the same pathogen is beaten faster, often before you feel ill. Vaccination uses this by showing the body a harmless form of the antigen first.

This guide goes from GCSE to A level in the order you would draw it. It is biology, not medical advice.

The one idea under everything: antigens

Every cell carries molecules on its surface, mostly proteins, that act like a name badge. These are antigens. Your own cells carry "self" antigens. A bacterium, a virus particle, or a cell infected by a virus carries "non-self" antigens. The immune system's job, at its simplest, is to recognise non-self and remove it.

Layer one: keeping pathogens out

These defences are non-specific: they work the same way against everything.

• Skin. A tough outer layer of dead cells that pathogens cannot easily cross.
• Mucus and cilia. Cells lining the airways produce sticky mucus that traps pathogens; tiny hairs called cilia sweep it up to the throat to be swallowed.
• Stomach acid. Hydrochloric acid in the stomach kills most pathogens in food and drink.
• Tears and saliva. These contain lysozyme, an enzyme that breaks down bacterial cell walls.

Layer two: the innate response

If a pathogen gets past the barriers, the innate response starts within minutes. It is still non-specific.

Phagocytosis. Phagocytes are white blood cells, such as neutrophils and macrophages, that engulf pathogens. The sequence, which A level exams ask you to describe in order:

1. The phagocyte is attracted to the pathogen by chemicals the pathogen releases or the damaged tissue releases.
2. It recognises the pathogen's non-self antigens and binds to it.
3. It engulfs the pathogen, enclosing it in a vesicle called a phagosome.
4. A lysosome fuses with the phagosome and releases digestive enzymes (lysozymes) that break the pathogen down.
5. A macrophage then displays the pathogen's antigens on its own surface. It is now an antigen-presenting cell, the bridge to layer three.

Inflammation. Damaged tissue and immune cells such as mast cells release histamine. Blood vessels nearby widen and become more permeable, so more blood, fluid and white blood cells reach the area. That is why a cut goes red, warm, swollen and sore.

Layer three: the adaptive response

The adaptive response is specific: each B or T cell recognises one particular antigen. It is slower the first time, because the right cells have to be found and multiplied, but it is far more powerful.

Both B and T cells are lymphocytes, made in the bone marrow. B cells mature in the bone marrow; T cells mature in the thymus.

T helper cells. A T helper cell with a receptor that fits the antigen on an antigen-presenting cell binds to it and is activated. It divides by mitosis and releases chemical signals called cytokines. Those signals stimulate B cells to divide, stimulate phagocytes to engulf more pathogens, and activate killer T cells.

B cells and antibodies (the humoral response). A B cell whose receptor fits the antigen binds to it. With signals from a T helper cell, it divides by mitosis to make a clone of identical cells. This is clonal selection and expansion. Most of the clone become plasma cells, which secrete huge numbers of antibodies. Some become memory B cells.

An antibody is a Y-shaped protein made of four polypeptide chains, two heavy and two light, held together by disulfide bridges. The tips of the Y are the variable region, which has a shape complementary to one specific antigen. Antibodies do not destroy pathogens on their own. They:

• Clump pathogens together (agglutination), so phagocytes can engulf many at once.
• Neutralise toxins, binding to them so they cannot harm cells (antibodies doing this are sometimes called antitoxins).
• Mark pathogens for phagocytes, making them easier to recognise and engulf.
• Block viruses from attaching to and entering body cells.

Killer T cells (the cell-mediated response). Once a virus is inside a body cell, antibodies cannot reach it. Infected cells display viral antigens on their surface. Cytotoxic (killer) T cells recognise those antigens and destroy the infected cell, for example by releasing perforin, which makes holes in its membrane. The virus loses the factory it was using to copy itself.

Every cell and its job

Why the second infection is faster

The first time you meet a pathogen, the primary response is slow. Only a few B and T cells fit its antigen, and they take several days to be selected and multiply. Meanwhile the pathogen is multiplying too, and you feel ill.

The second time, memory cells are already waiting in large numbers. The secondary response starts sooner, produces antibodies faster, and reaches a much higher level of antibody. The pathogen is usually destroyed before it causes symptoms. You are immune.

This is the graph exam boards love. Draw antibody concentration on the y axis against time on the x axis. The first exposure gives a small, late hump. The second exposure, weeks or years later, gives a tall, early spike. Label the delay before each, and label the second "faster, larger, longer lasting".

Vaccination: memory without the illness

A vaccine contains antigens from a pathogen in a form that cannot cause the disease: a dead or inactivated pathogen, a weakened (attenuated) one, a purified piece of it such as a surface protein, or genetic instructions (as in mRNA vaccines) that make your own cells produce one harmless viral protein for a short time.

The body treats it like a first infection. B and T cells that fit the antigen are selected and multiply, and memory cells are made. If the real pathogen arrives later, the fast secondary response destroys it. Some vaccines need booster doses to keep enough memory cells.

When enough people in a population are immune, a pathogen struggles to spread from person to person, which protects people who cannot be vaccinated. This is herd immunity.

Edward Jenner's 1796 experiment, in which he used material from cowpox to protect a boy against smallpox, is the classic history question. Smallpox was declared eradicated in 1980.

Active and passive immunity. Vaccination and infection give active immunity: your own body makes the antibodies and memory cells, so it lasts. Passive immunity is receiving antibodies made by someone else, such as a baby receiving antibodies across the placenta or in breast milk, or a person given an antibody treatment. It works at once but fades within weeks or months, because no memory cells are made.

Mistakes that lose easy marks

• Saying white blood cells "eat" viruses inside cells. Phagocytes engulf pathogens in the blood and tissues. Viruses inside cells are dealt with by killer T cells destroying the infected cell.
• Calling antibodies cells. They are proteins, made by plasma cells.
• Saying a vaccine contains antibodies. It contains antigens. Your body makes the antibodies.
• Saying antibiotics work on viruses. Antibiotics kill bacteria or stop them growing. They do nothing to viruses.
• Saying memory cells make antibodies. Memory B cells divide rapidly into plasma cells, and the plasma cells make the antibodies.

A scene plan for a five minute video

The immune response is a sequence with a cast, so follow one pathogen from start to finish. About 750 words of narration:

1. A cut on a finger and a bacterium with a distinctive antigen shape drawn on it.
2. Skin, mucus and acid as walls; most bacteria bounce off, a few get through the cut.
3. Inflammation: histamine, widening vessels, neutrophils squeezing out.
4. A macrophage engulfs one bacterium and holds its antigen up like a flag.
5. A T helper cell with the matching receptor binds, and cytokine signals go out as arrows.
6. One B cell fits; it clones into plasma cells pouring out Y-shaped antibodies, and a few memory cells set aside in a corner.
7. Antibodies clump the bacteria; phagocytes clear them up.
8. The antibody graph, primary hump drawn first.
9. Months later, the same bacterium returns; the memory cells react, and the secondary spike is drawn over the first.
10. Vaccination: the same story, but scene 1 is a syringe and nobody gets ill.

For more on a biology video built around one travelling thing, see how to make a biology explainer video for a school assignment, and for a health class version aimed at younger students, how to make a video for a health class.

Make your own version

1. 1

   Decide your level

   GCSE stops at barriers, phagocytes, antibodies, antitoxins, memory cells and vaccination. A level adds antigen presentation, T helper and killer T cells, clonal selection, antibody structure and the primary and secondary response graph.

2. 2

   Pick one pathogen and follow it

   A bacterium through a cut, or a virus through the airways. Every scene should move the story on one step for that one pathogen.

3. 3

   Write one paragraph per scene

   Use the scene plan above and the exact terms your specification uses. About 750 words is five minutes at roughly 150 spoken words a minute.

4. 4

   Paste the script into openCanviz

   Choose Keep my wording so terms such as cytokines and plasma cells are said as written, set a target length, and pick the whiteboard or cutout style so each cell can be a recognisable character.

5. 5

   Keep the cast consistent

   Give each cell type one look and keep it across scenes, so the macrophage in scene 4 is clearly the same kind of cell in scene 7. Recurring characters can be kept across a series if you make one video per topic.

6. 6

   Check every label and arrow

   Pause on each scene. Drafted drawings can show antibodies attacking a virus inside a cell or label a plasma cell as a memory cell. Fix any wrong label in the editor.

Common questions

What are the three lines of defence in the immune system? Physical and chemical barriers (skin, mucus, cilia, stomach acid), the innate response (phagocytes and inflammation), and the adaptive response (B cells, antibodies and T cells). The first two are non-specific; the third targets one particular pathogen.

What is the difference between an antigen and an antibody? An antigen is a molecule, usually a protein, on the surface of a cell or pathogen that the immune system recognises. An antibody is a protein made by plasma cells with a shape complementary to one antigen, so it binds to it.

Why do you not get chickenpox twice? After the first infection, memory cells for the chickenpox virus stay in the body. If you meet it again, the secondary response destroys it before it can cause the illness. The virus can stay dormant in nerve cells, though, and later cause shingles, which is a reactivation rather than a new infection.