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Adjuvant: In vaccines, a substance that is added to increase the immune response.

Antibody: A protein molecule that is produced by B cells in the body in response to contact with a foreign substance termed an antigen. Antibodies neutralize antigens and are an important part of the immune response against infection. Antibodies are also called immunoglobulins.

Antigen: A substance that can produce a specific immune response.

Attenuated: Weakened; in vaccines, the weakening of a virus or bacteria to disable its ability to cause disease.

Booster shot: A dose of a vaccine that helps the body maintain immunity against a disease.

Capsule: An anatomical structure surrounding the body of a bacteria.

FDA: Abbreviation of Food and Drug Administration, the U.S. government entity responsible for reviewing drugs and vaccines for effectiveness and safety.

Humoral immunity: Immunity referring to elements dissolved in the blood or body fluids, such as antibodies in the blood, rather than cells.

Lymph node: A small mesh of tissue that filters, attacks, and destroys harmful microbes. Part of a network of nodes scattered throughout the body to fight infection.

Lymphocyte: B cells or T cells that help the immune system fight an infection and provide humoral (B cell) or cellular (T cell) immunity.

Macrophage: A large white blood cell that kills invading microbes and takes the antigens of those cells to helper T cells, so that they can identify and kill the infection.

Membrane: A thin layer of tissue that covers a surface or divides a space within an organism.

Memory cell: T and B cells that control the ability of the immune system to remember a certain invader and prevent it from being able to cause infection in the future.

Mutate: To change in form, quality, or other characteristic.

Nonspecific immune system: A network of specialized cells, such as macrophages, that responds to infections at the same rate, whether it's a first invasion or a subsequent invasion.

Polysaccharide: A carbohydrate, or sugar, molecule found in the capsules of bacteria.

Protective immunity: Describes immunity at a high enough level to protect an individual from infection. Protective immunity can be attained as a result of a vaccine, or as a result of acquiring a particular disease.

Protein shell: Protective coating of protein molecules covering the body of a virus.

Specific immune system: Network of cells consisting of B and T cells and antigen-presenting cells that takes several days or more to respond to an infection the first time, but which responds more quickly and powerfully the next time.

Stabilizer: A chemical additive that helps a vaccine stay potent during shipping and storage.

Sterile: Free from living organisms or contaminants.

Toxin: A poison.