Serum Sickness

A type three, immune complex mediated hypersensitivity reaction occurring typically seven to twenty one days after exposure to a foreign protein or, in the case of serum sickness like reactions, certain non protein drugs, characterised by fever, rash, arthralgia and lymphadenopathy, historically associated with administration of animal derived antisera and now more commonly encountered with certain biologic therapies and antibiotics.

Pathophysiology

Formation of antibodies against the foreign antigen, occurring over one to two weeks following exposure, leads to circulating immune complex formation once sufficient antibody has developed relative to the persisting antigen. These complexes deposit in small vessels and tissues throughout the body, activating complement and triggering the characteristic pattern of inflammation, explaining the delayed onset relative to initial exposure and the self limiting course once the antigen is cleared from the circulation.

Causes

Animal derived antisera and antitoxins (rabies immunoglobulin, antivenom, antithymocyte globulin) are classical triggers, alongside certain monoclonal antibody biologic therapies (particularly those derived from non human sources or chimeric constructs) and antibiotics, particularly beta lactams and, historically, cefaclor in children, producing a serum sickness like reaction through a mechanism that may not always involve classical immune complex formation but produces a clinically similar picture.

Fever, typically developing seven to twenty one days after the initial exposure (or more rapidly, within one to three days, on re-exposure in a previously sensitised individual), accompanies the onset of other symptoms.

Rash, frequently urticarial or morbilliform, and often affecting the trunk and extremities, develops alongside arthralgia or arthritis, typically symmetrical and affecting multiple joints.

Lymphadenopathy, malaise and, less commonly, renal involvement (proteinuria, reflecting immune complex deposition in the glomeruli) complete the characteristic clinical picture.

Symptoms are generally self limiting, resolving over one to two weeks once the causative antigen has been cleared from the circulation and antibody antigen complex formation ceases.

A clear temporal relationship between the causative exposure and symptom onset, together with the characteristic combination of fever, rash and arthralgia, is central to recognising this condition.

Diagnosis is predominantly clinical, based on the characteristic history of exposure to a recognised trigger at an appropriate interval before symptom onset, together with the typical clinical picture.

Complement levels (C3 and C4) are frequently reduced during the active phase, reflecting consumption through immune complex mediated complement activation, and can support the diagnosis where measured during the symptomatic period.

Inflammatory markers are typically elevated, and urinalysis should be checked given the potential, albeit generally mild and self limiting, for renal involvement.

Skin biopsy, where performed for diagnostic uncertainty, may show a leukocytoclastic vasculitis pattern reflecting the underlying immune complex mediated process.

Exclusion of alternative diagnoses, particularly infection (which may itself have prompted the antibiotic exposure now under suspicion as the cause) and other causes of fever, rash and arthralgia, is an important part of the diagnostic process given the overlapping presentation with several other conditions.

Withdrawal of the causative agent, where ongoing exposure is occurring or further doses are planned, is the essential first step in management.

Symptomatic treatment with antihistamines for the rash and nonsteroidal anti inflammatory drugs for arthralgia and fever is sufficient for the majority of cases, given the generally self limiting nature of the condition.

Corticosteroids are reserved for more severe symptoms, significant renal involvement, or where symptoms fail to settle with simple symptomatic measures, typically given as a short tapering course.

The condition is generally self limiting, resolving over one to two weeks without long term sequelae once the causative antigen has cleared, and reassurance regarding this favourable natural history, alongside clear documentation of the causative agent to guide future avoidance, completes routine management.

Where the causative agent is a necessary and otherwise irreplaceable therapy (certain antivenoms or antitoxins in an emergency setting), premedication with antihistamines and corticosteroids, together with graded or slow administration, may be considered for future doses if re-exposure is unavoidable, under specialist guidance.