What clinical finding indicates serum sickness?

Serum sickness is a type of hypersensitivity reaction that can occur after exposure to certain medications or antigens, typically characterized by a set of clinical findings that arise due to immune complex formation. One of the hallmark indicators of serum sickness is lymphadenopathy, which results from the immune response to these circulating immune complexes. When the body detects foreign proteins or antibodies, it activates the immune system, leading to the proliferation of lymphocytes in lymph nodes, where the immune response is generated. This often manifests as enlarged lymph nodes, making lymphadenopathy a key clinical finding associated with serum sickness.

Other possible symptoms of serum sickness can include fever, rash, arthralgia (joint pain), and malaise, but lymphadenopathy is particularly notable as it reflects the activation of the lymphatic system in response to the antigen.