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[disambiguation needed] of complement by preformed circulating antibodies. In severe cases, the degree of complement fixation can be so substantial that it induces local tissue necrosis.