Zinc as a structural element of immune cells
Zinc is required by hundreds of enzymes and by zinc-finger transcription factors, so its influence on immunity is broad rather than confined to one pathway. It is needed for the maturation and function of T lymphocytes, including the thymic hormone thymulin which depends on zinc to be active, and it supports natural killer cell activity. Because zinc-finger proteins regulate gene expression in dividing immune cells, a shortage tends to blunt the proliferative response to infection.
Zinc deficiency is associated with thymic shrinkage and impaired cell-mediated immunity, which helps explain why deficient individuals are more susceptible to certain infections. Correcting a genuine deficiency restores these functions more reliably than supplementing people who already have adequate status.
The antiviral claim and why it is partly conditional
Laboratory work shows that raising intracellular zinc can inhibit the RNA-dependent RNA polymerase that several RNA viruses use to replicate, which is the basis of interest in zinc for respiratory infections. In practice, getting enough zinc inside the cell is the limiting factor, and clinical results (for example zinc lozenges and the common cold) are mixed, with effects that depend on formulation, dose, and timing. The direct enzyme inhibition is real in vitro but does not automatically translate into a reliable treatment effect.
This is general educational information, not medical advice. Excess zinc can impair copper status and immune function in its own right; consult a qualified healthcare professional before regular supplementation.