Adjuvant: A chemical added to a vaccine to boost immune response to an anti- 
genic component. 
Adverse effect: An undesirable, usually serious complication probably caused by 
the administration of a vaccine or a drug. 
Adverse event: Any undesirable experience that occurs in a defined period after 
the administration of a vaccine or a drug. The vaccine or drug does not have 
to be the cause Of the event. 
Anaerobic: A bacterium that grows in the absence of oxygen (e.g., the bacterium 
that causes tetanus or botulism). 
Anaphylaxis: A serious, life-threatening reaction. In hypersensitive individuals, 
anaphylaxis has causes as diverse as bee venom, peanuts, shellfish, and anti- 
gens In some vaccines. 
Animal reservoir: Species of animals that can be infected by a virus that also 
infects humans so that the virus cannot be eradicated. 
Antibody: A protein molecule (immunoglobulin) produced by B cells in response 
to a specific antigen. 
Antibody titer test: A laboratory procedure for determining the strength Of anti- 
body production against an antigen. 
Antigen: A nonself molecule that induces an immune system response. resulting 
in the production of antibodies. 
Antigen-presenting cell: An immune system cell that engulfs a pathogen and 
then exhibits some of the pathogen's protein on its surface so that the antigen- 
presenting cell looks like a foreign cell. Macrophages and dendritic cells are 
the major antigen-presenting cells. 
Attenuated: Said ofa whole virus or bacterium that has been grown under adverse 
conditions or chemically treated so that it is weakened to the point where it 
does not cause the diseuse but stimulates an adequate immune response to it. 
B cell: An antibody-producing cell that is part of the adaptive immune system. 

Background rate: The frequency with which an adverse event occurs in an 

unvaccinated population. This rate is compared to the frequency of the same 

adverse event in a comparable vaccinated population to help determine if the 

vaccine causes an increase in occunence of the adverse event. 

Biologic product: A medical product made from natural sources such as human 

Or animal cells, or microorganisms. 

Blood serum: The clear yellowish fluid that remains after blood cells and clotting 

factors have been removed from whole blood. 

Bone marrow: Soft tissue that fills the hollow spaces of bones. Red bone mar- 

row produces stem cells that become erythrocytes (red blood cells), lymphocytes (white blood cells), and platelets. Yellow bone marrow produces stem 

cells that become fat, bone, and cartilage. Red and yellow bone marrow exist 

within the same bone cavities. 

Clinical trial: A research study using humans in which volunteers are assigned 

to various groups to test the safety and effectiveness of a drug before it is 

licensed for public use. 

Clonal expansion: The ability of a cell to divide and create millions of identical 

daughter cells. 

Cold chain: A continuous system of cold boxes, refrigerators, and freezers used 

to keep biological products such as vaccines within a narrow temperature 

range during transport, distribution, and storage to prevent loss of potency 

of the product. 

Cytokines: Small signaling proteins released by cells that control the behavior of 

other cells and help mediate the immune response. 

Encephalitis: Acute swelling and inflammation of the brain that can result in 

permanent brain damage. 

Formalin: A solution of formaldehyde and water. 

Guillain-Barré syndrome (GBS): A rare syndrome in which the immune system 

attacks healthy nerve cells resulting in weakness or tingling sensations that 

move up the body and can cause paralysis. GBS appears to increase after 

infection with the Zika virus. There has been some controversy over whether 

influenza vaccination also increases the incidence of GBS. 

Hematopoietic stem cells: Stem cells in the bone marrow that have the potential 

to differentiate into many different types of blood cells. 

Herd immunity: Also called community immunity, this condition occurs when a 

high-enough percentage of the population is immune to a contagious disease 

through vaccination or naturally acquired immunity that there is little oppor- tunity for the disease to spread to nonimmune individuals. Immune tolerance: The ability of the immune system to recognize and leave unharmed an individual's self cells while attacking foreign cells and material. Immunity: In medicine, the ability of an organism to resist disease, usually through the development of antibodies against the disease-causing pathogen. In law, exemption from certain legal proceedings, such as being sued Or pros ecuted for a crime.

Secondary response: Stimulation of antibody production by memory cells on re- 

encountering a pathogen, usually taking two to seven days. Rapid secondary 

response is the basis for active immunity. 

Spleen: The largest lymphoid organ in the body. Located to the left of the stomach and below the diaphragm, it produces lymphocytes (white blood cells) 

and removes old and damaged red blood cells from the blood. 

Stabilizer: Substance added to a vaccine to shield it against temperature changes and to maintain its potency and extend its shelf life. 

Subunit vaccine: A vaccine made by identifying and isolating the pathogen's 

antigenic proteins without including any of the pathogen's genetic material. 

T cell: One of several related cells that mature in the thymus and help direct the chain of reactions that result in adaptive immunity. 

Thymus: A butterfly-shaped lymphatic organ essential to the production of 

T cells. It gradually decreases in size after puberty. 

Vector: An organism, often a biting insect or tick, that transmits a disease from 

one animal to another or from an infected animal to a human. 

Zoonoses: Diseases that normally exist in animals but that can be transmitted to humans and cause illness,