Plasma is the clear fluid portion of blood. It does not contain blood cells. However, 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 an infection or poisonous substance. Antiserum may be used to protect a person who has been exposed to a germ he or she has not been vaccinated again.
For example, you may receive a certain type of antiserum injection if you have been exposed to tetanus or rabies. This is called passive immunization. It gives you immediate, but temporary, protection while your body develops an active immune response against the toxin or germ.
During serum sickness, the immune system falsely identifies a protein in antiserum as a potentially harmful substance (antigen). The result is an immune system response that attacks the antiserum. Immune system elements and the antiserum combine to form immune complexes, which cause the inflammation and other symptoms of serum sickness.
Certain medications (such as penicillin, cefaclor, and sulfa) can cause a similar reaction. Unlike other drug allergies, which occur very soon after receiving the medication, serum sickness develops 7 - 21 days after the first exposure to a medication.
Injected proteins such as antithymocyte globulin (used to treat organ transplant rejection) and rituximab (used to treat immune disorders and cancers) can cause serum sickness reactions.
Blood products may also cause serum sickness.