On the right, a green pathogen studded with antigens has slipped past the skin. Two defenders move in: a large phagocyte that will engulf it (phagocytosis), and a B-lymphocyte that makes Y-shaped antibodies. ▶ Play to watch the phagocyte close in and the antibodies bind the antigens — the lock-and-key fit that marks the invader for destruction.
Look closely at the antibodies pouring from the B-lymphocyte. Each is a Y shape, and its two tips fit one specific antigen like a key in a lock. ⏭ Step through to see them latch on: this clumps the pathogens together, neutralises their toxins, and labels them so phagocytes destroy them even faster. ↻ Reset and ▶ Play again to repeat the whole attack.
Switch to the Vaccination tab. A vaccine carries a weakened antigen — enough to be recognised, not enough to cause illness. ▶ Play: it triggers a small, slow primary response of antibodies, and — the key point — it leaves behind long-lived memory cells. Watch the left, low curve build: that is the body learning the enemy safely.
Keep watching the same graph. ⏭ Step on, and the real pathogen arrives. Now the memory cells act at once: the secondary response is far faster and higher than the first — see the tall right-hand curve shoot up. The pathogen is destroyed before symptoms appear, so the person stays well. That is why one childhood jab protects for years.
Open the Disease & antibiotics tab. A colony of bacteria is causing disease, and most are green and vulnerable — but one, by chance mutation, is a red resistant cell. ▶ Play: the antibiotic kills every non-resistant bacterium. The infection seems cured… but watch what survives.
The lone resistant survivor now has the field to itself. ⏭ Step through the next cycles: it divides and multiplies until, by natural selection, the whole colony is resistant and the antibiotic no longer works — a "superbug". ↻ Reset to replay. The lesson: only use antibiotics when needed, and always finish the full course. Revisit the full story in the Lecture.