Serum is the clear fluid portion of blood. It does not contain blood cells, but it 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 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 an allergy. The immune system misidentifies a protein in the 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. Blood products may also induce serum sickness.

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

• Skin rash
• Itching
• Hives
• Joint pain
• Fever
• Malaise
• Enlarged lymph nodes

The goal of treatment is the relief of symptoms.

Topical corticosteroids or other soothing topical (applied to a localized area of the skin) medications may relieve discomfort from itching and rash. Antihistamines may shorten the duration of illness and help to relieve rash and itching.

Nonsteroidal anti-inflammatory drugs (NSAIDs) may relieve joint pain. Corticosteroids such as prednisone may be prescribed for severe cases.

Medications causing the problem should be stopped and future use of the medication or antiserum should be avoided.

Health care providers (such as dentists and hospital personnel) should be advised of drug allergies before treating the patient. Identifying jewelry or cards (such as Medic-Alert or others) may be advised.

Increased risk of anaphylaxis for future exposures to the substance is a possible complication.