An Arthus reaction is a localized immune-complex tissue reaction caused by injecting a large amount of antigen into tissue where a large amount of antibody is present. The reaction peaks within a few hours and is characterized by neutrophils responding to the immune complex and causing inflammation. Serum sickness arises when serum from a horse other animal is used in the preparation of antibodies for injection into a human (passive immunity); the recipient’s immune system mounts a response to antigens in the foreign serum.