You got some things mixed up here :)

Stem cells are the origin of all cells, the baseline.

Dendrites (like neurons have them!) are "the branched projections of a neuron that act to propagate the electrochemical stimulation"

Dendritic cells are immune cells, as described in this post. They have dendrites too but those are non-neuronal dendrites.

Dendrite comes from the greek δένδρον or déndron meaning tree, so in this case it describes the long "arms" dendritic cells develop.

I hope I could clear up the confusion!