an allergic (immune system disorder) reaction to animal serum or antiserum injected into an individual's blood to provide immunity against such illnesses as tetanus, botulism, and snake-venom poisoning. Symptoms include skin eruption, itching, swelling of the face and extremities, fever, joint pain and sometimes swelling, abdominal pain, nausea, and vomiting; severe cases may also show neurological symptoms.

The disorder arises when the patient's antibodies (antibody) attack the animal serum's proteins as they would any foreign invader, forming antigen-antibody complexes that lodge in the blood vessel walls. complement, a series of blood proteins, is then activated, causing inflammation. Serum sickness develops within two weeks of serum injection and usually lasts only a few days. Its severity depends on both the amount of serum injected and the previous immune state of the patient; sensitized patients react more quickly than those producing antibodies for the first time. Also, persons with a history of allergy are more likely to develop serum sickness.