An injection of antiserum (passive immunization) may be used when a person has been exposed to a potentially dangerous microorganism against which the person has not been immunized. It provides immediate, but temporary protection while the person develops a personal immune response against the toxin or microorganism. Examples include antiserum for tetanus and rabies exposure.

Serum sickness is a hypersensitivity reaction similar to allergy. The immune system misidentifies a protein in antiserum as a potentially harmful substance (antigen), and it develops an immune response against the antiserum.

Antibodies bind with the antiserum protein to create larger particles (immune complexes). The immune complexes are deposited in various tissues, causing inflammation and various other symptoms.

Because it takes time for the body to produce antibodies to a new antigen, symptoms do not develop until 7 to 21 days after initial exposure to the antiserum. Patients may develop symptoms in 1 to 3 days if they have previously been exposed to the offending agent.

Exposure to certain medications (particularly penicillin) can cause a similar process. Unlike other drug allergies, which occur very soon after receiving the medication for the second (or subsequent) time, serum sickness can develop 7 to 21 days after the first exposure to a medication. The drug molecules probably combine with a protein in the blood before being misidentified as an antigen.

Blood products may also induce serum sickness.

Serum sickness is different from anaphylactic shock, which is an immediate reaction with more severe symptoms.

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