spell immunity - different types?

What is the difference between certain types of spell immunity?

I noticed that slow poison can affect some spell immune units.

sorry for posting twice.. the button wouldn't load up the next page.. but it posted twice.. is there a way to delete it?

There's no major difference between the different spell imuneties except from the icon and that the ''spell imunety (dragon)'' doesn't work (I'm not sure about the last, but till now I haven't found any spells that dragons are imune to).

The only difference is that the icon for Spell Imunity appears in different slots...
I beleive it is:

Dryad: 1st slot in 3rd row.
Dragon: 2nd slot in 3rd row
Archimonde: 2nd slot in 2nd row.

I didn't know spell imunity (Dragon) did not work! In pre-1.03 version of the game, Dragons were immune to spells. Later, such imunity was removed. I imagined that Blizzard just removed the ability, I didn't know it was totally disabled... that's bad...

The archimonde spell immunity is immune to spells and vampiric aura's life leech. While at least one of the others is immune to spells but you can also still leech life off the unit.