Full mechanism, in text

Training immune T cells

1 · Thymus · T-cell academy

T precursor cells from the bone marrow pass two selection gates in the thymus, positive and negative, before maturing and leaving, and this training is highly zinc-dependent.The thymus sits behind the sternum and is the only site for T-cell maturation. T precursor cells (CD4⁻CD8⁻) from the bone marrow enter the thymus and pass through two gates in the cortex and medulla:

· Positive selection — those that recognise self-MHC survive
· Negative selection — those that strongly recognise self-antigen are killed (to avoid autoimmunity)

Survivors mature into CD4⁺ (helper T) or CD8⁺ (killer T) and leave the thymus into blood and lymph.

Key: this training is highly zinc-dependent.

2 · Thymulin · Zn²⁺ activated

The thymus secretes thymulin, which is active only when it binds one Zn²⁺, so without zinc it stays inactive, the thymus shrinks and T-cell output falls.Thymic epithelial cells secrete a small peptide called thymulin (9 amino acids) — it must bind one Zn²⁺ to be active:

Thymulin + Zn²⁺ → active Zn-thymulin

Active thymulin is a key signal for T-cell differentiation, and several enzymes in T-cell activation signalling also need zinc.

Zinc deficient → thymulin cannot be activated → the thymus shrinks → T-cell output falls — in animal experiments, zinc deficiency makes the thymus shrink quickly.

3 · Zn deficit → thymic atrophy

In animal experiments zinc deficiency shrinks the thymus, kills more T precursor cells and weakens antibody responses; in people, those short of zinc get repeated infections more easily.When zinc runs short (mostly from animal experiments):

· The thymus shrinks
· More T-precursor cells die by apoptosis
· The CD4/CD8 balance is disturbed
· Antibody responses weaken (B cells also depend on zinc)

In people: those who are zinc deficient get repeated infections more easily.

Older adults: the thymus wastes with age (immunosenescence), T-cell output falls and vaccine responses weaken. It has been proposed that long-term low zinc is part of the cause — this is still a hypothesis, and whether zinc supplements can turn it back is not settled.

4 · Cold lozenges · a different matter

Cold lozenges are a different matter: their free zinc acts mainly in the nose and throat, not by raising whole-body zinc; treatment evidence is low-certainty and prevention does little.The current Cochrane review (Nault 2024):
· Treatment-dose zinc shortens illness by about 2.4 days on average (95% 0.5-4.2), but the studies vary enormously (I² = 97%) and certainty is low
· For preventing colds it has little or no effect
· Narrowed to the three zinc acetate lozenge with the right form, dose and timing (Hemilä 2016 individual-patient , N = 199): about 2.7 days shorter than the 7-day average; all three started within 24 hours and used 80-92 mg/day of elemental zinc

Lozenges versus swallowed tablets:
· Lozenges: zinc acetate or zinc gluconate, dissolved slowly in the mouth — the proposed action is local (the mechanism is still a hypothesis)
· Swallowed tablets: by the local mechanism they should not help, but this has never been compared directly; for correcting whole-body deficiency, swallowing is the right route

Dose reminder: the cold dose is only for the few days of the cold. The adult tolerable upper intake level () for zinc is 40 mg/day, and going above it long term blocks copper absorption — this is not a long-term tonic.

Full mechanism, in text

1 · Thymus · T-cell academy

2 · Thymulin · Zn²⁺ activated

3 · Zn deficit → thymic atrophy

4 · Cold lozenges · a different matter

Thymus · T-cell maturation

Full mechanism, in text

1 · Thymus · T-cell academy

2 · Thymulin · Zn²⁺ activated

3 · Zn deficit → thymic atrophy

4 · Cold lozenges · a different matter

Thymus · T-cell maturation