Immune reaction which can occur after the injection of animal serum, formerly used for vaccination. The body identifies proteins in the animal serum as alien, causing an allergic and immune reaction. The disorder occurs if serum protein is present in excessive proportion to the antibodies which the body makes against it. Symptoms can include fever, swollen lymph nodes, skin eruptions or painfully swollen joints. Symptoms occur about 8 days after the injection, sooner and more severely if the patient has previously been in contact with the serum. Nowadays serum sickness is rare because most vaccines are purified or prepared synthetically.