HUMAN · IMMUNOLOGY

THE IMMUNE SYSTEM

ACT 01 · THE STREAM

inside a venule, drawn to scale

You are inside a small vein about 30 micrometres across. Every red cell here is the real 7.8 µm disc, thinner in the middle than at the rim. Blood runs fastest down the centre and slowest at the wall — the parabolic profile of any liquid in a pipe — so the middle cells overtake the edge ones. The paler, larger cells creeping along the wall are neutrophils: they catch on adhesion molecules and roll at micrometres a second while everything else streams past at about a millimetre a second.

1 unit = 1 µm · vessel 30 µm across · slowed ~100×

where this comes from

Education only, not medical advice. The scenes are drawn, but drawn to scale — one unit is one micrometre — except the antibodies, which are enlarged about two hundred times to be visible. Speeds are slowed or sped up as each act says. The antibody curves show the textbook shape of primary and memory responses; real numbers vary by pathogen and person.

  • red blood cell 7.8 µm across, ~2.5 µm thick at the rim
  • platelet 2–3 µm
  • neutrophil 12–15 µm, 3–5 nuclear lobes
  • macrophage ~21 µm
  • lymphocyte (T, B) ~7 µm
  • bacterium (E. coli) ~2 µm long, 0.5–1 µm wide
  • antibody (IgG) ~10–15 nm — drawn ~200× bigger
  1. Murphy K, Weaver C. Janeway's Immunobiology, 9th ed. Garland Science, 2016 — innate response, leukocyte rolling and diapedesis, antigen presentation, primary vs secondary responses.
  2. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology, 10th ed. Elsevier, 2021 — kinetics of primary and secondary antibody responses (IgM then IgG; memory responses faster and larger).
  3. Evans E, Fung YC. Improved measurements of the erythrocyte geometry. Microvasc Res 1972;4:335–347 — the red cell's biconcave shape used for every disc here.
  4. Dancey JT et al. Neutrophil kinetics in man. J Clin Invest 1976;58:705–715; Summers C et al. Neutrophil kinetics in health and disease. Trends Immunol 2010;31:318–324 — roughly 10¹¹ neutrophils made per day.
  5. Sender R, Milo R. The distribution of cellular turnover in the human body. Nat Med 2021;27:45–48 — about 3.3×10¹¹ cells replaced daily, most of them red cells.
  6. Alberts B et al. Molecular Biology of the Cell, 6th ed. Garland Science, 2015 — a plasma cell secretes on the order of 2,000 antibody molecules per second.
  7. Krombach F et al. Cell size of alveolar macrophages. Environ Health Perspect 1997;105(Suppl 5):1261–1263 — ~21 µm.
  8. Sezonov G, Joseleau-Petit D, D'Ari R. Escherichia coli physiology in Luria-Bertani broth. J Bacteriol 2007;189:8746–8749 — ~20 min doubling in ideal conditions (slower in a real wound).
  9. Guyton AC, Hall JE. Textbook of Medical Physiology — blood velocity in capillaries and venules on the order of a millimetre per second.

EDUCATION ONLY · NOT MEDICAL ADVICE