serum sickness, hypersensitive response that occurs after injection of a large amount of foreign protein. The condition is named for the serum taken from horses or other animals immunized against a particular disease, e.g., tetanus or diphtheria. Such serum, which contains antibodies against the disease toxins, was formerly widely used to temporarily immunize humans. However, the antibodies from the animal serum are also foreign proteins that can act as antigens when injected into humans. The recipient's body responds by producing, within 8 to 12 days, antibodies that react against the animal serum proteins; the reaction causes injury to blood vessel walls and such allergic symptoms as rash, itching, and swelling of the lymph nodes. Fever, joint pain, spleen enlargement, and even shock may occur (see allergy; hypersensitivity). The reaction subsides as continued production of antibodies removes foreign protein from circulation. A person who has once had a serum injection is sensitized to the serum antigens, and a second injection can bring on the acute reactions typical of anaphylaxis. Today, serum preparations are rarely used. Instead, inoculations of tetanus and diphtheria toxoids are given in childhood; they confer active immunity against those diseases. Serum sickness may occur in response to proteins other than those found in serum.