= Nutritional immunity
\b[Nutritional immunity] is a host-defense strategy that limits the supply of <nutrients> available to a <pathogen>. Restricting access to needed metal <ions> is one example: a host <protein> can bind a metal so that its presence in tissue does not imply that invading <bacteria> can acquire it.
A concrete experimental example is <calprotectin> in <Staphylococcus aureus> <abscesses>. <Brian D. Corbin> and colleagues found that <neutrophil>-derived <calprotectin> inhibited bacterial growth through <chelation> of <manganese> and <zinc>. Abscesses in mice lacking <calprotectin> contained more metal and supported greater bacterial proliferation. The combination of growth inhibition and the comparison with deficient animals connects metal sequestration to a host defense, rather than merely showing that a binding <protein> is present at an infection site.<Metal chelation and inhibition of bacterial growth in tissue abscesses by Brian D. Corbin and others>[Abstract; neutrophil-dependent calprotectin, Mn2+/Zn2+ restriction, and the comparison with calprotectin-deficient mice]{ref}
A simple two-pool model makes the distinction between total amount and accessibility explicit. Write the total metal <concentration> as $M_T=M_{\mathrm{free}}+M_{\mathrm{bound}}$. If $M_T=1\,\mu\mathrm{mol\,L^{-1}}$ and $99\%$ is bound, the free <concentration> is only $0.01\,\mu\mathrm{mol\,L^{-1}}$: one hundredth of the value if all the metal were free. These are illustrative numbers, not measurements from the experiment. Actual microbial access also depends on whether a <bacterium> can obtain metal from the bound pool; total metal <concentration> alone therefore does not measure nutrient availability.
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