The Antibodies are a race of dangerous robots that act as 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 authorised.
Antibodies resemble jellyfish, with long, dangling "tentacles" protruding from a main "head". They appear in blue shades, with the "head" a darker shade than the rest. When not activated, they are stored in the walkways of the Teselecta and defend the robot from intruders.
The main function of an Antibody is to purge the Teselecta, a chameleonic android-like robot designed to punish criminals, of 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 Antibodies, along with the Teselecta, appear in the episode "Let's Kill Hitler" (Season 6, Episode 8). In the episode, they capture and kill the Nazi Zimmerman, and almost do the same to Amy and Rory, but they were given authorization bracelets by a crew member and were therefore saved. After the Teselecta begins to punish Melody for her crime of killing the Doctor, Amy and Rory use the sonic screwdriver to disable the crew's bracelets, causing the Antibodies to attack them. The crew escapes and takes back control of the robot by the episode "The Wedding of River Song", by which time it is implied that the Antibodies have been deactivated or reprogrammed.