שמות יב ,ב: וְרָאִיתִי אֶת־הַדָם וּפָסַחְתִי עֲלֵכֶם וְלֹ א־יִהְיֶה בָכֶם נֶגֶף לְמַשְחִית בְּהַכֹתִי בְּאֶרֶץ מִצְרָיִם
The Sforno understands this verse that in addition to the plague of killing the first-born other plagues occurred that night and the blood on the doorposts protected the Jews.
Since HaShem had stated that He would exact the punishment of the firstborn on that night, what possible danger could be for the Jews that they had to be confided to their houses and place blood on the doorposts?
The Sforno understands that many additional plagues were visited on the Egyptians that night as listed in Tehilim 78. Exposure to these plagues would have endangered even the Jews since those plagues were done by the angels.
