In medicine, a booster dose is an extra administration of a vaccine after an earlier dose. After initial immunization, a booster injection or booster dose is a re-exposure to the immunizing antigen cell. It is intended to increase immunity against that antigen back to protective levels after it has been shown to have decreased or after a specified period. For example, tetanus shot boosters are often recommended every 10 years. If a patient receives a booster dose but already has a high level of antibody, then a reaction called an Arthus reaction could develop, a localized form of Type III hypersensitivity, induced by fixation of complement by preformed circulating antibodies. In severe cases, the degree of complement fixation can be so substantial that it induces local tissue necrosis.