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adjuvants—substances sometimes included in a vaccine formulation to enhance the immune-stimulating properties of a vaccine.

antibodies—molecules produced by a B cell in response to an antigen. When an antibody attaches to an antigen, it helps destroy the microbe bearing the antigen.

antigen—a molecule on a microbe that identifies it as foreign to the immune system and stimulates the immune system to attack it. artificially acquired immunity—immunity provided by vaccines, as opposed to naturally acquired immunity, which is acquired from exposure to a disease-causing organism.

attenuation—the weakening of a microbe so that it can be used in a live vaccine.

B cells—white blood cells crucial to the immune defenses. Also known as B lymphocytes, they come from bone marrow and develop into blood cells called plasma cells, which are the source of antibodies.

bacteria—microscopic organisms composed of a single cell and lacking a defined nucleus and membrane-enclosed internal compartments.

booster shots—supplementary doses of a vaccine, usually smaller than the first dose, that are given to maintain immunity.

cell-mediated immune response (also called cellular immune response)—immune protection provided by the direct action of immune cells (as distinct from that provided by molecules such as antibodies).

clinical trial—an experiment that tests the safety and effectiveness of a vaccine or drug in humans.

complement proteins—molecules that circulate in the blood whose actions “complement” the work of antibodies. Complement proteins destroy antibody-coated microbes.

conjugate vaccine—a vaccine in which proteins that are easily recognizable to the immune system are linked to the molecules that form the outer coat of disease-causing bacteria to promote an immune response. Conjugate vaccines are designed primarily for very young children because their immune systems can not recognize the outer coats of certain bacteria.

contagious—able to transmit disease to other people.

cytotoxic T cells (also called killer T cells)—a subset of T cells that destroy body cells infected by viruses or bacteria.

DNA vaccine (also called naked DNA vaccine)—a vaccine that uses a microbe’s genetic material, rather than the whole organism or its parts, to simulate an immune response.

edible vaccines—foods genetically engineered to produce antigens to specific microbes and safely trigger an immune response to them.

Food and Drug Administration (FDA)—the Federal agency that approves and licenses vaccines and drugs.

formalin—a solution of water and formaldehyde, used in toxoid vaccines to inactivate bacterial toxins.

genetic material—molecules of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) that carry the directions that cells or viruses use to perform a specific function, such as making a particular protein molecule.

genomes—all of an organism’s genetic material. A genome is organized into specific functional units called genes.

Haemophilus influenzae type b (Hib)—a bacterium found in the respiratory tract that causes acute respiratory infections, including pneumonia, and other diseases such as meningitis.

helper T cells—a subset of T cells that function as messengers. They are essential for turning on antibody production, activating cytotoxic T cells, and initiating many other immune functions.

herd immunity (also called community immunity)—the resistance to a particular disease gained by a community when a critical number of people are vaccinated against that disease.

humoral immune response (also called antibody response)— immune protection provided by B cells, which secrete antibodies in response to antigen (as distinct from that provided by the direct action of immune cells, or the cellular immune response).

immune—having a high degree of resistance to or protection from a disease.

immune system—a collection of specialized cells and organs that protect the body against infectious diseases.

inactivated vaccine (also called “killed” vaccine)—a vaccine made from a whole viruses or bacteria that has been inactivated with chemicals or heat.

live, attenuated vaccine—a vaccine made from microbes that have been weakened in the laboratory so that they can’t cause disease. (See attenuation.)

lymph nodes—small bean-shaped organs of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are gathering sites of B, T, and other immune cells.

lymphocytes—white blood cells that are central to the immune system’s response to foreign microbes. B cells and T cells are lymphocytes.

macrophages—large and versatile immune cells that devour and kill invading microbes and other intruders. Macrophages stimulate other immune cells by presenting them with small pieces of the invaders.

memory cells—a subset of T cells and B cells that have been exposed to antigens and can then respond more readily and rapidly when the immune system encounters the same antigens again.

microbe—a microscopic organism. Microbes include bacteria, viruses, fungi, and single-celled plants and animals.

molecules—the building blocks of a cell. Some examples are proteins, fats, and carbohydrates.

mutate—to change a gene or unit of hereditary material that results in a new inheritable characteristic.

naked DNA vaccines—(See DNA vaccines.)

naturally acquired immunity—immunity produced by antibodies passed from mother to fetus (passive), or by the body’s own antibody and cellular immune response to a disease-causing organism (active).

pertussis (also called whooping cough)—a respiratory infection caused by the toxic bacterium Bordetella pertussis. The wracking coughs characteristic of this disease are sometimes so intense the victims, usually infants, vomit or turn blue from lack of air.

Phase IV studies—very large clinical studies that look at tens of thousands of people who have received a licensed vaccine. These studies try to pick up rare or delayed adverse reactions that might not have been apparent in the smaller Phase I, II, and III studies that preceded licensure.

plasma cells—cells produced by dividing B cells that are entirely devoted to producing and secreting antibodies.

polysaccharides—long, chain-like molecules made up of linked sugar molecules. The outer coats of some bacteria are made of polysaccharides.

preclinical testing—required laboratory testing of a vaccine before it can be given to people in clinical trials. Preclinical testing is done in cell cultures and in animals.

recombinant DNA technology—the technique by which genetic material from one organism is inserted into a foreign cell or another organism in order to mass-produce the protein encoded by the inserted genes.

recombinant subunit vaccines—vaccines made using recombinant DNA technology to engineer the antigen molecules of the particular microbe. (See subunit vaccine.)

recombinant vector vaccines—vaccines that use modified viruses or bacteria to deliver genes that code for microbial antigens to cells of the body.

rubella (also called German measles)—a viral disease often affecting children and spread through the air by coughs or sneezes. Symptoms include a characteristic rash, low-grade fever, aching joints, runny nose, and reddened eyes. If a pregnant woman gets rubella during her first three months of pregnancy, her baby is at risk of having serious birth defects or dying.

subunit vaccine—a vaccine that uses one or more components of a disease-causing organism, rather than the whole, to stimulate an immune response.

T cells—white blood cells (also known as T lymphocytes) that direct or participate in immune defenses. (See cytotoxic T cells and helper T cells.)

toxin—agent produced by plants and bacteria, normally very damaging to cells.

toxoids (also called inactivated toxins)—toxins, such as those produced by certain bacteria, that have been treated by chemical means, heat, or irradiation and are no longer capable of causing disease.

toxoid vaccine—a vaccine containing a toxoid, used to protect against toxins produced by certain bacteria.

Vaccine Adverse Events Reporting System (VAERS)—a follow-up surveillance system that gathers information on adverse reactions associated with licensed vaccines. VAERS is a joint effort of FDA and CDC, and anyone—doctors, patients, or parents—can report adverse reactions to VAERS.

vector—in vaccine technology, a bacterium or virus that cannot cause disease in humans and is used in genetically engineered vaccines to transport genes coding for antigens into the body to induce an immune response.

virulent—toxic, causing disease.

viruses—very small microbes that do not consist of cells but are made up of a small amount of genetic material surrounded by a membrane or protein shell. Viruses cannot reproduce by themselves. In order to reproduce, viruses must infect a cell and use the cell’s resources and molecular machinery to make more viruses.

Commonly Used Abrreviations

i.m. intramuscular injection
s.c. subcutaneous injection
i.p. intraperitoneal injection
i.n. intranasal immunization
CFU Colony Forming Unit