The mark that spares a person is the life-force consciously placed at the threshold of the body. Blood is the vital spirit itself, and set upon the doorposts it signs the house so that the destroying force sweeping through the flesh passes it by. The self that has raised its inner life to a visible token is not touched by the death moving through the lower realm.
The houses are the bodies of those awake, and Egypt is the whole domain of matter under judgment. The plague is the dissolution that overtakes everything unmarked, everything still identified only with the perishing outer form. What reads as an ancient night of terror is the passing over of that in a person which has lifted its own life above the flesh, while all that clings to matter falls.
Blood: the vital spirit and life-force of a person, raised and placed as a conscious sign
The houses: the bodies of those awake, marked and set apart from the surrounding death
Egypt: the entire domain of matter and the flesh, now under judgment
The plague: the dissolution that overtakes all that is still identified only with the perishing outer form