The Antibodies are a race of dangerous robots that act as minor antagonists in the Doctor Who universe - though they are technically following program rather than being malicious by choice, they are nevertheless heartless killers that treat living beings as foreign bodies to be destroyed unless they are authorized.
Antibodies resemble jellyfish, with long, dangling "tentacles" protruding from the main head. They appear in shades of blue, with the head a darker shade than the rest. When not activated, they are stored in the walkways of the Teselecta. When an intruder is detected, they will rise out of the walkway and proceed toward the intruder.
The main function of an Antibody is to purge the Teselecta, a chameleonic android-like robot designed to punish criminals, or unwanted visitors, intruders, and the criminals they capture. When an Antibody detects an intruder, it will announce itself by saying, "Welcome. You are unauthorized. Your death will now be implemented." Other messages include "Welcome. You will experience a tingling sensation and then death", "Remain calm while your life is extracted", and other variations of the same. If the intruder is given an authorization bracelet from a crew member after the Antibodies have been activated, they will stop pursuing their target and say, "You are authorized. Your existence will continue."
Appearance in Doctor Who
The Teselecta, along with the Antibodies, appear in Let's Kill Hitler, where the Antibodies capture and kill the Nazi Erich Zimmerman. When Amy and Rory are teleported onto the ship, the Antibodies initially pursue them, but the two are given authorization bracelets by a crew member, effectively saving them. After the Teselecta begins to punish River Song for her crime of killing the Doctor, Amy and Rory use his sonic screwdriver to deactivate the crew's bracelets, setting the Antibodies lose on them. Eventually, the crew escapes and successfully takes back control of the robot by the episode The Wedding of River Song, by which time it is implied that the Antibodies have either been deactivated or reprogrammed.