Serum sickness is a type of immune reaction that occurs when the body produces antibodies against certain foreign substances like medicines, vaccines, or antiserum. Some of the synonyms for serum sickness include type III hypersensitivity reaction, serum illness, serum reaction, antiserum sickness, and drug-induced serum sickness. These phrases are typically used interchangeably depending on the context and underlying cause of the immune response. Regardless of the terminology used, serum sickness often presents with flu-like symptoms such as fever, joint pain, and rash, and requires immediate medical attention to prevent serious complications.