Serum sickness

Definition:

A group of symptoms caused by a delayed immune response to certain medications or antiserum (passive immunization with antibodies from an animal or another person)

Causes, incidence, and risk factors:

Serum is the clear fluid portion of blood. It does not contain blood cells but does contain many proteins, including antibodies, which are formed as part of the immune response to protect against infection.

Antiserum is a preparation of serum that has been removed from a person or animal that has already developed immunity to a particular microorganism. It contains antibodies against that microorganism. An injection of antiserum (passive immunization) may be used when a person has been exposed to a 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 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 14 days after exposure to the antiserum.

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 14 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.

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