serum sickness, an immunological disorder that may occur 2 to 3 weeks after the administration of an antiserum. It is caused by an antibody reaction to an antigen in the donor serum. The condition is characterized by fever, splenomegaly, swollen lymph nodes, skin rash, and joint pain. Treatment is symptomatic and supportive and may include corticosteroids. See also angioedema, antigen-antibody reaction, Arthus reaction.