  We explore the elemental abundances in Galactic planetary nebulae (PNe)
compared with those of their stellar progenitors (Red Giant Branch and
Asymptotic Giant Branch, RGB and AGB, stars), to explore and quantify the
expected -- i.e., due to AGB evolution or condensation onto grains --
differences. We gleaned the current literature for the nebular abundances while
we used the APOGEE DR~17 survey data for the stellar sample. We examined the
elements in common between the nebular and stellar samples, namely, C, N, O,
Fe, and S. We confirm that iron in PNe is mostly entrapped in grains, with an
average depletion $<$D[Fe/H]$>$=1.741$\pm$0.486 dex, and we disclose a weak
correlation between iron depletion and the [O/H] abundance,
D[Fe/H]$=(6.6003\pm2.443)\times{\rm [O/H]} +(1.972\pm0.199)$. Sulfur may also
be mildly depleted in PNe, with $<$D[S/H]$>=0.179\pm0.291$ dex. We also found
an indication of nitrogen enrichment for PNe $<$E[N/H]$>$=0.393$\pm$0.421 dex,
with maximum enrichment (0.980$\pm$0.243) occurring for the PNe whose
progenitors have gone through the HBB. The carbon enrichment is
$<$E[C/H]$>$=0.332$\pm$0.460 dex when measured for the general PN populations.
Our results will be relevant for future Galactic and extragalactic studies
comparing nebular and stellar samples.
