Serum sickness is an unusual reaction to any foreign substance in the body. Venom from insect stings or spider bites and medicines such as penicillin are common causes of this reaction.

Symptoms of serum sickness usually begin between 7 to 10 days after the person is exposed to the substance. A person usually feels generally unwell (malaise) and may have hives, joint pain, fever, headache, and swollen glands.

Having an episode of serum sickness puts a person at high risk for developing a severe allergic reaction if he or she is exposed to the same substance in the future. A person should avoid any medicine related to serum sickness after it has been identified. Venom immunotherapy may be an option to protect against insect or spider bites that caused the reaction.