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Mechanism
Intermediate
9 min read

The immune system

A defence that recognises before it fights, and whose main tool — inflammation — is not a malfunction. Two pages of this corpus invoke it without any of them describing it.

A defence that starts by recognising

The problem this system solves is not first of all to fight: it is to distinguish. It has to tell what belongs to the organism from what does not, otherwise every attack would turn against the body itself.
That recognition works at two levels, which do not operate the same way.

Two ways of defending

Level
What it recognises
Delay
Memory
Innate
General signatures, shared by whole families of threats
Immediate
None
Adaptive
A precise target, identified afterwards
Days
Lasting
The innate defence is present from the start and responds within minutes, without ever having met the threat. It is fast and coarse.
The adaptive defence takes days to build, but it recognises a precise target and keeps that recognition. This is why a second encounter with the same threat produces a faster and stronger response.

Inflammation is not a malfunction

This is the most useful correction on the page, and it illuminates two others in this corpus.
Inflammation
A local response to an aggression or an injury: the vessels dilate, become more permeable, and defence cells flow towards the area. It produces heat, redness, swelling and tenderness.
inflammatory reaction
Those four signs are not the damage: they are the means. Dilation brings blood, permeability lets cells through, and increased tenderness protects the area by making it unpleasant to use.
The muscle soreness page describes a delayed pain whose causal chain is not established, mentioning an inflammatory response. This page says what that word covers — and why observing it is not enough to establish that it is the cause.
The problem, then, is not inflammation but its persistence. A response that does not switch off matches exactly the logic described on the stress page: what wears you down is not the reaction, it is the failure to end it.

Exercise and defences: a field that changed its mind

Here is a subject calling for particular caution, because what used to be taught has been revised.
Certainty level · Probable
Regular moderate physical activity is associated with better immune defence, while the idea of a window of vulnerability opening after intense effort has been strongly re-examined.
The review gathers several decades of work in exercise immunology. It describes a robust association for regular activity, while noting that the classical interpretation of the cellular changes observed after intense effort — long read as a transient immunosuppression — is now debated. These are associations across populations, not a guaranteed effect in one person.
Nieman DC, Wentz LM (2019)
Caution
What this page will not say
No food, no supplement and no practice is presented here as "boosting immunity". The literature cited does not support that kind of claim, and the vocabulary itself misleads: a system that must both respond and hold back is not "strengthened" the way a muscle is.
Body Lab moreover addresses no disease, no vaccination, no allergy and no immune disorder.

What sleep does about it

The Recovery pillar takes on an extra meaning here. The sleep and recovery page describes a period of restoration, and the stress page a response whose switching off matters more than its triggering.
The immune system sits at the intersection of the two: it is modulated by the same mediators as the stress response, which McEwen's review describes under the term allostatic load. A prolonged constraint that never subsides therefore acts on more than the heart and metabolism.

Why no scene accompanies this page

Limit
A system with no shape
The other systems pages declare either an absence of data or a problem of scale. Here a third reason is added: this system has no shape. Its cells circulate in the blood and the lymph, its tissues are scattered, and part of its action happens at any point in the body.
None of the anatomical sources Body Lab uses contains it, and there is no view that would show it honestly.

What this page does not do

Caution
No disease, no interpretation
This page describes a general functioning. It assesses no individual situation and allows no symptom to be interpreted.
A fever that lasts, repeated infections, a lymph node that persists, a wound that does not heal or an allergic reaction belongs to a health professional. A severe allergic reaction — swelling of the face or throat, difficulty breathing — is an emergency.

Sources

Main sources

  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Nieman DC, Wentz LM (2019). The compelling link between physical activity and the body's defense system. Journal of Sport and Health Science.
  • McEwen BS (1998). Protective and damaging effects of stress mediators. The New England Journal of Medicine.
Put it into practice in Shapier

Go deeper into chronic inflammation and training

Following up in practice on inflammation linked to training happens in Shapier; Body Lab only explains what inflammation is and why its persistence is a problem.
Go deeper into chronic inflammation and training
Body Lab explains; Shapier lets you act and track.

Check my understanding

Heat, redness, swelling and tenderness: what are these four signs of inflammation?
Why does a second encounter with the same threat produce a faster and stronger response?
What does research say about the window of vulnerability thought to open after intense effort?
Choose an answer

Read next

  • Stress
    A mobilisation response, not an illness. What wears you down is not triggering it — it is never switching it off, and training is itself one of the triggers.
    Described without a scene
  • Delayed muscle soreness
    The ache that arrives the next day is not lactic acid. What is known about where it comes from, how it runs its course, and why it measures neither the quality nor the value of a session.
    With a 3D scene
  • Sleep and recovery
    What does sleep actually do for recovery? This page describes the architecture of a night, what sleep deprivation changes in a documented way, and clearly separates established results from hypotheses that are still open.
    With a 3D scene

Available offline

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • No scene accompanies this page: the immune system is spread through the whole body, its cells circulate, and none of the anatomical sources Body Lab uses contains it.
  • The relationship between exercise and defences is a field where conclusions have changed: several long-taught claims are now debated, and the page says so.
  • This page addresses no disease, no vaccination, no allergy and no immune disorder.
  • No supplement, no food and no practice is presented as boosting immunity: the literature cited does not support it.
Sources
  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Nieman DC, Wentz LM (2019). The compelling link between physical activity and the body's defense system. Journal of Sport and Health Science.
  • McEwen BS (1998). Protective and damaging effects of stress mediators. The New England Journal of Medicine.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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