You need to choose a marker that you can "afford" to use for destruction - which either doesn't exist at all in healthy cells, or, more likely, it exists but lo

You need to choose a marker that you can "afford" to use for destruction - which either doesn't exist at all in healthy cells, or, more likely, it exists but losing all your healthy cells with the same marker won't kill you (but is still likely to harm you, just in ways that can be treated). Such markers are scarce, and not guaranteed to exist; if we find one for this cancer (like CD19) that works, but if it fails, then it's exceedingly likely that targeting "cd21 or another target" will simply kill you because it will also destroy some vital cells in your liver or veins or brain.