The Dalek antibodies were robotic guards inside the Dalekanium casing of a Dalek.
They served as security, exterminating any foreign elements that intruded and caused damage and/or hurt the Dalek. Inside of "Rusty", they killed both Ross and Gretchen Carlisle. Much like a biological organism can't control their antibodies, Dalaks couldn't control theirs. (TV: Into the Dalek)