User blog comment:FloaterJ/Nitpicker's Nook/@comment-26131697-20150222152610/@comment-25176701-20150222154944

To add on to number 2, it isn't just bone marrow that needs to match up, but the blood that the MM get from the Grounders has to be the same blood type as them so they don't get killed by it. However, I'm not so sure the MM check the blood types of the Grounders because they didn't check Bellamy's (when he was pretending to be a Grounder).