DefinitionSerum sickness is a reaction similar to an allergy. Specifically, it is an immune system reaction to certain medications, injected proteins used to treat immune conditions, or antiserum, the liquid part of blood that contains antibodies that help protect against infectious or poisonous substances.See also: Immune responseCauses, incidence, and risk factorsPlasma 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 produced from the plasma of a person or animal that has immunity against a particular infection or poisonous substance. Antiserum may be used to protect a person who has been exposed to a potentially dangerou