Innate Immunity: The First Wall
Skin, mucus, fever and cells that eat intruders — the defense you were born with responds in minutes and treats every germ the same.
Builds on: 0.1 What Is Alive, and What a Cell Is0.3 Enzymes: Why Biology Happens at All
Most infections never happen
You inhale, swallow and touch millions of microbes a day, and almost none of them make you ill. That is not luck; it is a layered defense that starts before anything gets in. Skin is a dry, slightly acidic wall of dead cells that is constantly shed. Mucus traps whatever lands in your airways, and beating cilia sweep it out. Tears and saliva carry lysozyme — an enzyme, in the Unit 0 sense, whose active site happens to fit and break bacterial cell walls. Stomach acid finishes most of what you swallow.
Everything on that list works against any microbe. That is the signature of the innate immune system: fast, ancient, and completely generic. It recognises broad patterns — “this looks bacterial”, “this looks like a virus” — never individual species.
When something gets through: eat it
Past the barriers, the front-line responders are phagocytes — “eating cells” — chiefly macrophages stationed in your tissues and neutrophils that pour in from the blood. They engulf microbes whole and digest them with enzymes. A single macrophage can eat about a hundred bacteria in its lifetime; a serious infection is a numbers race between bacterial doubling and phagocyte clearance, and the lab below lets you run exactly that race.
Inflammation is your doing, not the germ’s
An infected cut goes red, hot, swollen and sore. Every one of those is a defensive act. Damaged cells and sentries release histamine, which widens local blood vessels (red, hot) and makes their walls leaky (swollen) so that phagocytes and antibacterial proteins can flood out of the blood into the tissue. The soreness is local nerves being sensitised so you protect the site. Inflammation feels like the disease; it is actually the fire brigade.
Fever: fighting with the thermostat
The same logic scales up to the whole body. Immune signals reset the brain’s thermostat a couple of degrees higher, and Unit 0.3 tells you why that helps: rates of enzyme-run processes are temperature-sensitive. Many pathogens are tuned to 37 °C and divide more slowly at 39 °C, while several of your own defensive processes speed up. A moderate fever is not damage — it is deliberately tilting a race you intend to win.
Pus is mostly dead neutrophils — millions of short-lived eating cells that rushed in, engulfed what they could, and died at their posts. A visibly messy wound is often evidence of a defense that worked.
The limits of asking no questions
The innate system is fast precisely because it is generic — nothing needs to be learned. But generic has a ceiling: it cannot get better at fighting this particular pathogen, and some invaders evolve past its standard tricks. For those, you need a system that learns. That is the next lesson.
⚗️ Lab — Growth vs. Clearance
Bacteria double; phagocytes eat. Whoever compounds faster wins.
- With a 2 h doubling time, find the fewest phagocytes that still clear the infection.
- Set a fast doubler (0.5 h). Watch it overwhelm a defense that was winning easily before.
- Now switch on fever. Slower germs, faster defenders — see which lost races it flips.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.