22.2 Two Arms of Defense

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Hours versus weeks

Two kinds of illness live in her chart and they keep different clocks. The flares arrive in hours: a vague unease, then a fever that climbs past 39, then one serous membrane on fire — the belly, the chest, a knee — for a day or three, then nothing. They have looked the same since she was two. The infections are different. Strep at 17 and at 37, each answered six weeks later by a shower of teardrop psoriasis; shingles the January she dropped out of school; a respiratory virus at 42 that most adults shake off in a week and that kept her for six. Her flares are too fast and tidy to be infections; her infections are too slow for a healthy host. Two arms, two clocks, two threads.

See it move: Scene 1 · Two Arms · Scene 5 · Interferon, Complement, CRP — and a Missing Lectin

II · Three lines and a clock

The lecture builds defense in three lines. The first is the barriers: skinThe body’s largest organ, providing protection and regulation. with its keratinA strong, fibrous protein that forms the structure of skin, hair, and nails. and acid mantleA thin, slightly acidic film on the skin that acts as a barrier to bacteria and contaminants. (dermcidin, sebumAn oily secretion that helps keep the skin and hair moisturized., a pHA measure of hydrogen ion concentration in a solution. near 5), the four mucous membranes washed in mucus, ciliaHair-like projections on the surface of some cells that move fluids or particles., lysozyme, stomach acidA substance that releases hydrogen ions (H⁺) in solution. and secretory IgA, and the normal flora that crowd out invaders. The second is innate immunity: cells and proteins born knowing a few hundred patterns — bacterial walls, flagellin, viral RNA, the chemistry of a damaged cell — read by pattern-recognition receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter such as the Toll-like receptors, answering in minutes to hours, the same way every time. Its cells come off the myeloid branch of the hematopoietic tree: neutrophils (the microphages, first to the wound, dead in the pus), monocytes that become macrophages with local names — Kupffer, alveolar, microgliaSmall CNS glial cells that act as macrophages, cleaning up debris and pathogens., osteoclastBone-resorbing cells that break down bone matrix., histiocyte — eosinophils, basophils and mast cells with their histamine and heparin, dendritic cellsImmune cells in the epidermis that help fight infections. that carry the news to the node, and the natural killer cell, a lymphocyte on innate rules. Its proteins are interferons, complement, C-reactive protein and the mannose-binding lectin. The third line is adaptive immunity: B and T lymphocytes with ten to the fourteenth possible receptors, each cell knowing one face, slow, precise, and — what the innate arm cannot do — remembering. The lecture puts the two on one clock: five to seven days for a pathogen to establish, two to three days of recognition, about two weeks of attack, then memoryThe ability to store and recall information. for decades; innate holds the line for the first four or five days. A fever that peaks at twelve hours and is gone by seventy-two has not waited for the adaptive arm. It was never invited.

See it move: Scene 2 · The First Line · Scene 3 · The Innate Cells, Tree to Smear · Scene 5 · Interferon, Complement, CRP — and a Missing Lectin

III · Phagocytosis, and the proteins that help

The innate arm’s signature act is phagocytosis, and the Innate Cells lecture teaches it as five steps: chemotaxis along a gradient to the site, adherence (easier when the target is opsonized — coated with complement C3b or antibody, the “condiment” that makes a bacterium tasty), ingestion into a phagosome, fusion with a lysosomeAn organelle filled with digestive enzymes that break down waste and cellular debris. to make a phagolysosome, and digestion, with the respiratory burst — hydrogen peroxide, superoxide, hypochlorite — doing the killing and leaving the macrophage’s own granules spent. Around the cells sit the antimicrobial proteins. Interferons, released by virus-infected cells, warn their neighbours to make antiviral enzymesProteins that speed up chemical reactions in the body. and call NK cells. Complement is twenty-odd plasmaThe liquid component of blood. proteins made by the liverA large organ that produces bile, detoxifies blood, and stores nutrients. and always present: the alternative pathway fires spontaneously on microbial surfaces; the classical pathway fires when C1 meets an antibody already bound; the lectin pathway fires when mannose-binding lectin grips the sugars on a bacterial or viral coat. All three converge on C3 — C3b opsonizes, C3a and C5a call phagocytes and degranulate mast cells, and C5 through C9 drill the membrane-attack complex through a bacterial wall. C-reactive protein, made by the liver within hours of an IL-6 signal, binds bacterial walls and dying cells and lights the classical pathway; it is also the acute-phase marker every one of her flares pushes up. Her 2015 workup found the lectin low. MBL deficiency is common, usually silent, and in some people it means ordinary viruses overstay — a six-week RSV, say — because the lectin pathway never got the first grip. That is a separate innate thread from the FMF fire, and the course keeps it separate; but it sits in the same arm, and it is the reason this chapter will not let “your immune system is fine” stand.

See it move: Scene 4 · Phagocytosis, Step by Step · Scene 5 · Interferon, Complement, CRP — and a Missing Lectin

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: The innate arm — over-firing, and short one protein, Neither on its own, or A weak or self-attacking adaptive arm. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.