Researchers at the University of Wisconsin-Madison and the Scripps Research Institute in California took blood samples from 19 volunteers who had been taking opioid painkillers for chronic lower back pain. They then screened these samples for antibodies to opioids… In 10 of these patients, they found opioid-specific antibodies.
…[T]hese antibodies could be part of the reason why people who take opioids can develop symptoms such as hyperalgesia, where a person’s sensitivity to pain becomes so extreme that even normally harmless sensations became painful.
The authors also speculate that these native immune responses may decrease the effectiveness of potential vaccines for opioid use disorder. Some scientists, including the authors of the study, are hoping to create vaccines that can induce a temporary antibody response to the opioids we get from a prescription pill or illicit drug. The antibodies would cling to the molecules from these drugs when they enter the system, blocking them from getting into the brain and causing the effects associated with opioid abuse, such as an addictive euphoria.