Does Defense matter when a card says deal damage?

Hummerhorns - Forced: After Hummerhorns engages you, deal 5 damage to a single hero you control.

Does my hero's defense block any of that damage?

What if I optionally engaged it? Does that still come into effect?

It doesn't matter if you optionally engage with him or if he engages with you, the effect still produces. See second last paragraph from page 15 from the core rules.

The defense doesn't count, so Hummerhorns deals 5 damage. It pretty much kills all heroes you can choose, except Beorn (Hobbit)

CJMatos said:

and Frodo (raise your threat level instead of damaging him)