Serum is the clear fluid portion of blood. While it does not contain blood cells, it does contain many proteins, including antibodies, which are formed as part of the immune response to help protect against infections.
Antiserum refers to a sample of serum from either a person or animal that has developed immunity to particular foreign substances (antigens). Antiserum contains antibodies against that particular antigen.
If an individual is exposed to a potentially dangerous foreign substance that he is not immune to, an injection of antiserum may be administered. This treatment provides temporary, but immediate protection until the person develops his or her own immune response against the antigen. For instance, antiserums for tetanus and rabies exposure are commonly used.
Serum sickness is a hypersensitive reaction, which occurs in response to certain antiserums. The body's immune system mistakes a protein in the antiserum as an antigen, and white blood cells attack it. The body then develops an immune response against the antiserum.
Allergic reactions can be classified into four immunopathologic categories. These classifications are based on the immune system's response to the allergen, not on the severity of the reaction. Serum sickness is a Type III allergic reaction, which involves the formation of an antigen-antibody immune complex that deposits on blood vessel walls and activates cell components known as complements. In most cases of serum sickness, immunoglobulin G (IgG) antibodies mediate the response. Immunoglobulin M (IgM) complexes are less common and elicit immune complexes more readily removed by the reticuloendothelial system. Immunoglobulin E-mediated vasoactive amines (substance containing amino groups that alters blood vessels' permeability or causes vasodilation) may cause hives and may further initiate complement activation.
Patients who have serum sickness usually recover completely within 7-28 days. Fatalities are rare and usually are the result of continued exposure to the antigen.
The frequency of serum sickness appears to be dose related. In one study, 10% of patients who received 10mL of tetanus antitoxin developed serum sickness, while nearly all patients who received 80mL or more developed serum sickness. All of the patients were successfully treated.
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