In medicine, a booster dose is an extra administration of a vaccine

Vaccine

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

 after an earlier dose. After initial immunization

Immunization

Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an agent ....

, a booster injection or booster dose is a re-exposure to the immunizing antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

. 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 of time. For example, tetanus

Tetanus

Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers. The primary symptoms are caused by tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped, obligate anaerobic bacterium Clostridium tetani...

 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

Arthus reaction

In immunology, the Arthus reaction is a type of local type III hypersensitivity reaction. Type III hypersensitivity reactions are immune complex-mediated, and involve the deposition of antigen/antibody complexes mainly in the vascular walls, serosa , and glomeruli.-History:The Arthus reaction was...

 could develop, a localized form of Type III hypersensitivity

Hypersensitivity

Hypersensitivity refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. These reactions may be damaging, uncomfortable, or occasionally fatal. Hypersensitivity reactions require a pre-sensitized state of the host. The four-group classification...

, induced by fixation

Fixation

Fixation may refer to the following:In science:*Fixation , the state in which an individual becomes obsessed with an attachment to another human, an animal, or an inanimate object...

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